Changing The value of struct in an array

Besides what said by @MikeS, remember that structs are value types. So in the for loop:

for test in testings {

a copy of an array element is assigned to the test variable. Any change you make on it is restricted to the test variable, without doing any actual change to the array elements. It works for classes because they are reference types, hence the reference and not the value is copied to the test variable.

The proper way to do that is by using a for by index:

for index in 0..<testings.count {
    testings[index].value = 15
}

in this case you are accessing (and modifying) the actual struct element and not a copy of it.

How to change Array of Structs

for every element:

itemsArray.indices.forEach { itemsArray[$0].someValue = newValue }

for specific element:

itemsArray.indices.filter { itemsArray[$0].propertyToCompare == true }
                  .forEach { itemsArray[$0].someValue = newValue }

To simplify working with value types in arrays you could use following extension (Swift 3):

extension Array {
    mutating func modifyForEach(_ body: (_ index: Index, _ element: inout Element) -> ()) {
        for index in indices {
            modifyElement(atIndex: index) { body(index, &$0) }
        }
    }

    mutating func modifyElement(atIndex index: Index, _ modifyElement: (_ element: inout Element) -> ()) {
        var element = self[index]
        modifyElement(&element)
        self[index] = element
    }
}

Example usage:

testings.modifyElement(atIndex: 0) { $0.value = 99 }
testings.modifyForEach { $1.value *= 2 }
testings.modifyForEach { $1.value = $0 }

Well I am going to update my answer for swift 3 compatibility.

When you are programming many you need to change some values of objects that are inside a collection. In this example we have an array of struct and given a condition we need to change the value of a specific object. This is a very common thing in any development day.

Instead of using an index to determine which object has to be modified I prefer to use an if condition, which IMHO is more common.

import Foundation

struct MyStruct: CustomDebugStringConvertible {
    var myValue:Int
    var debugDescription: String {
        return "struct is \(myValue)"
    }
}

let struct1 = MyStruct(myValue: 1)
let struct2 = MyStruct(myValue: 2)
let structArray = [struct1, struct2]

let newStructArray = structArray.map({ (myStruct) -> MyStruct in
    // You can check anything like:
    if myStruct.myValue == 1 {
        var modified = myStruct
        modified.myValue = 400
        return modified
    } else {
        return myStruct
    }
})

debugPrint(newStructArray)

Notice all the lets, this way of development is safer.

The classes are reference types, it's not needed to make a copy in order to change a value, like it happens with structs. Using the same example with classes:

class MyClass: CustomDebugStringConvertible {
    var myValue:Int

    init(myValue: Int){
        self.myValue = myValue
    }

    var debugDescription: String {
        return "class is \(myValue)"
    }
}

let class1 = MyClass(myValue: 1)
let class2 = MyClass(myValue: 2)
let classArray = [class1, class2]

let newClassArray = classArray.map({ (myClass) -> MyClass in
    // You can check anything like:
    if myClass.myValue == 1 {
        myClass.myValue = 400
    }
    return myClass
})

debugPrint(newClassArray)

You have enough of good answers. I'll just tackle the question from a more generic angle.

As another example to better understand value types and what it means they get copied:

struct Item {
    var value:Int

}

func change (item: Item, with value: Int){
    item.value = value //  cannot assign to property: 'item' is a 'let' constant
}

That is because item is copied, when it comes in, it is immutable — as a convenience.

Had you made Item a class type then you were able to change its value.

var item2 = item1 // mutable COPY created
item2.value = 10
print(item2.value) // 10
print(item1.value) // 5

This is very tricky answer. I think, You should not do like this:

struct testing {
    var value:Int
}

var test1 = testing(value: 6)
var test2 = testing(value: 12)

var ary = [UnsafeMutablePointer<testing>].convertFromArrayLiteral(&test1, &test2)

for p in ary {
    p.memory.value = 3
}

if test1.value == test2.value {
    println("value: \(test1.value)")
}

For Xcode 6.1, array initialization will be

var ary = [UnsafeMutablePointer<testing>](arrayLiteral: &test1, &test2)

It is possible to use the map function to get this effect - essentially creating a new array

itemsArray = itemsArray.map { 
  var card = $0
  card.isDefault = aCard.token == token
  return card
} 

I ended up recreating a new array of struct see the example below.

func updateDefaultCreditCard(token: String) {
    var updatedArray: [CreditCard] = []
    for aCard in self.creditcards {
        var card = aCard
        card.isDefault = aCard.token == token
        updatedArray.append(card)
    }
    self.creditcards = updatedArray
}

I tried Antonio's answer which seemed quite logical but to my surprise it does not work. Exploring this further I tried the following:

struct testing {
    var value:Int
}

var test1 = testing(value: 6 )
var test2 = testing(value: 12 )

var testings = [ test1, test2 ]

var test1b = testings[0]
test1b.value = 13

// I would assume this is same as test1, but it is not test1.value is still 6

// even trying 

testings[0].value = 23

// still the value of test1 did not change.
// so I think the only way is to change the whole of test1

test1 = test1b