How could I change the text once I click on button ?

There is 2 parts which are needed for your question

The first part is as jimmson stated in their solution; use the innerText property to set the value.

But this will not work with your code as is; as there is nothing calling your ShowHelloMessage() function. You will need to add an event property to your button to say what to do when the button is clicked

<button id="clickMe" onclick="ShowHelloMessage();">Click Me</button>

You will notice the empty parenthesis in both the function and the event calling it. You could place a variable name in the function itself there and pass that value on the button itself

<script>
function ShowHelloMessage(NewMessageValue) {
	var GetText = document.getElementById("clickMe");
	GetText.innerText = NewMessageValue;
}
</script>
<button id="clickMe" onclick="ShowHelloMessage('Mike was here!');">Click Me</button>