Using innerHTML is SO NOT RECOMMENDED. Instead, you should create a textNode. This way, you are "binding" your text and you are not, at least in this case, vulnerable to an XSS attack.

document.getElementById("myspan").innerHTML = "sometext"; //INSECURE!!

The right way:

span = document.getElementById("myspan");
txt = document.createTextNode("your cool text");
span.appendChild(txt);

For more information about this vulnerability: Cross Site Scripting (XSS) - OWASP

Edited nov 4th 2017:

Modified third line of code according to @mumush suggestion: "use appendChild(); instead".
Btw, according to @Jimbo Jonny I think everything should be treated as user input by applying Security by layers principle. That way you won't encounter any surprises.

EDIT: This was written in 2014. A lot has changed. You probably don't care about IE8 anymore. And Firefox now supports innerText.

If you are the one supplying the text and no part of the text is supplied by the user (or some other source that you don't control), then setting innerHTML might be acceptable:

// * Fine for hardcoded text strings like this one or strings you otherwise 
//   control.
// * Not OK for user-supplied input or strings you don't control unless
//   you know what you are doing and have sanitized the string first.
document.getElementById('myspan').innerHTML = 'newtext';

However, as others note, if you are not the source for any part of the text string, using innerHTML can subject you to content injection attacks like XSS if you're not careful to properly sanitize the text first.

If you are using input from the user, here is one way to do it securely while also maintaining cross-browser compatibility:

var span = document.getElementById('myspan');
span.innerText = span.textContent = 'newtext';

Firefox doesn't support innerText and IE8 doesn't support textContent so you need to use both if you want to maintain cross-browser compatibility.

And if you want to avoid reflows (caused by innerText) where possible:

var span = document.getElementById('myspan');
if ('textContent' in span) {
    span.textContent = 'newtext';
} else {
    span.innerText = 'newtext';
}

document.getElementById('myspan').innerHTML = 'newtext';

I use Jquery and none of the above helped, I don't know why but this worked:

 $("#span_id").text("new_value");

Here's another way:

var myspan = document.getElementById('myspan');

if (myspan.innerText) {
    myspan.innerText = "newtext";
}
else
if (myspan.textContent) {
        myspan.textContent = "newtext";   
}

The innerText property will be detected by Safari, Google Chrome and MSIE. For Firefox, the standard way of doing things was to use textContent but since version 45 it too has an innerText property, as someone kindly apprised me recently. This solution tests to see if a browser supports either of these properties and if so, assigns the "newtext".

Live demo: here

In addition to the pure javascript answers above, You can use jQuery text method as following:

$('#myspan').text('newtext');

If you need to extend the answer to get/change html content of a span or div elements, you can do this:

$('#mydiv').html('<strong>new text</strong>');

References:

.text(): http://api.jquery.com/text/

.html(): http://api.jquery.com/html/

Many people still come across this question (in 2022) and the available answers are not really up to date.

Use innerText is the best method

As you can see in the MDM Docs innerText is the best way to retrieve and change the text of a <span> HTML element via Javascript.

The innerText property is VERY well supported (97.53% of all web users according to Caniuse)

How to use

Simple retrieve and set new text with the property like this:

let mySpan = document.getElementById("myspan");

console.log(mySpan.innerText);

mySpan.innerText = "Setting a new text content into the span element.";

Why better than innerHTML ?

Don't use innerHTML to updating the content with user inputs, this can lead to major vulnerability since the string content you will set will be interpreted and converted into HTML tags.

This means users can insert script(s) into your site, this is known as XSS attacks/vulnerabilities (Cross-site scripting).

Why better than textContent ?

First point textContent isn't supported by IE8 (but I think in 2022 nobody cares anymore). But the main element is the true difference of result you can get using textContent instead of innerText.

The example from the MDM documentation is perfect to illustrate that, so we have the following setup:

<p id="source">
  <style>#source { color: red;  } #text { text-transform: uppercase; }</style>
<span id=text>Take a look at<br>how this text<br>is interpreted
       below.</span>
  <span style="display:none">HIDDEN TEXT</span>
</p>

If you use innerText to retrieve the text content of <p id="source"> we get:

TAKE A LOOK AT
HOW THIS TEXT
IS INTERPRETED BELOW.

This is perfectly what we wanted.

Now using textContent we get:

  #source { color: red;  } #text { text-transform: uppercase; }
Take a look athow this textis interpreted
       below.
  HIDDEN TEXT

Not exactly what you expected...

This is why using textContent isn't the correct way.

Last point

If you goal is only to append text to a <p> or <span> HTML element, the answer from nicooo. is right you can create a new text node and append it to you existing element like this:

let mySpan = document.getElementById("myspan");

const newTextNode = document.createTextNode("Youhou!"),

mySpan.appendChild(newTextNode);

You may also use the querySelector() method, assuming the 'myspan' id is unique as the method returns the first element with the specified selector:

document.querySelector('#myspan').textContent = 'newtext';

developer.mozilla

Like in other answer, innerHTML and innerText are not recommended, it's better use textContent. This attribute is well supported, you can check it this: http://caniuse.com/#search=textContent

There is a pretty basic method to change the inner text of any HTML tag using DOM. Use document.querySelector to find your span, like so document.querySelect('#span') notice that #span just like a CSS selector.

After getting your tag the best option is to use "innerText" to change the text of the tag. There is a second option is to use "textContent" to change the text.

const span = document.querySelector("#span");
const btn = document.querySelector("#changeBtn");

btn.addEventListener("click", () => {
span.innerText = "text changed"
})

<span id="span">Sample Text</span>
<button id="changeBtn">Change Text</button>

document.getElementById("myspan").textContent="newtext";

this will select dom-node with id myspan and change it text content to new text

You can do

 document.querySelector("[Span]").textContent = "content_to_display"; 

Can't be used with HTML code insertion, something like:
var a = "get the file <a href='url'>the link</a>"
var b = "get the file <a href='url'>another link</a>"
var c = "get the file <a href='url'>last link</a>"

using
document.getElementById("myspan").textContent=a;
on
<span id="myspan">first text</span>

with a timer but it just shows the reference target as text not runing the code, even tho it does shows correctly on the source code. If the jquery approch is not really a solution, the use of:

document.getElementById("myspan").innerHTML = a to c;
is the best way to make it work.

For this span

<span id="name">sdfsdf</span>

You can go like this :-

$("name").firstChild.nodeValue = "Hello" + "World";

(function ($) {
    $(document).ready(function(){
    $("#myspan").text("This is span");
  });
}(jQuery));

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<span id="myspan"> hereismytext </span>

user text() to change span text.

I used this one document.querySelector('ElementClass').innerText = 'newtext';

Appears to work with span, texts within classes/buttons

For some reason, it seems that using "text" attribute is the way to go with most browsers. It worked for me

$("#span_id").text("text value to assign");