Don't spend too many horses on this.
You need to first look at how jQuery's selector works. It works similar to CSS selectors (if you're not familiar with that I suggest you start with something more basic).
If you need a quick review on jQuery syntax. In your example you need to use the element selector $('button') and then you'll want to apply the .text() function to change the text for the button. So if you put it together. You'll want to select the button and then apply the text() function, passing in the string you want to change the text to, to change it's text.
$('button').text('Insert Text Here');