You would first need to set a listener to your button, say our markup was as follows:

<button id='btn'>test it</button>

<span id='label'>Test</span>

We can select the button element, and set it's onclick attribute:

document.getElementById('btn').onclick = test

Then we can define a function call test that will toggle the label:

function test(){

 document.getElementById('label').innerHTML = 'Tested';

}

And that's it!

Here's a jsfiddle demonstrating this code.