Instead, you need to set the innerHTML 'property:
function changeText() { document.getElementById('myButton').innerHTML= "New value"; }
You can specify a value on the button, but it is not used very often. In your case, you want to change the button text. So innnerHTML is your friend. See this page for more details.
Also note that you can also use "innerText" in IE, but it is not supported in Firefox (and probably not in some others). "textContent" may also be an option, but that is not supported in older browsers (until 2011). Thus, innerHTML is the safest option.
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