We can manipulate any DOM element using its id as selector together with document.getElementById. The advantage is that we can manipulate the inner html part of the element.
Let's see some example.
Try the following example...
Should I explain it? It looks quite simple... Try it for yourself and see how by clicking on the button the inner HTML of the div is changed.
document.getElementById('theElem').innerHTML = '... enjoy yourself';
<div id='theElem'>Welcome to the Web Thought...</div>
<input type='button' onclick='changeIH()' value='What's next?'/>
This is different. Let's change the text according to a user input:
Fill in the input box and press the button: the text "Chose a new text..." is replaced with the new text.
var userText = document.getElementById('userText').value;
document.getElementById('theElem').innerHTML = userText;
<div id='theElem'>Chose a new text...</div>
<input type='text' id='userText' value='New Text' />
<input type='button' onclick='changeIH()' value='Submit'/>
As you can see we can really do anything we want and manipulate the HTML inside a specific tag.
I could go on with many more example however I think we got to the point.
I leave the rest to your imagination.