this is pretty simple, but i always forget it so ill add it here

document.getElementById("theid").innerHTML = "the new content";

heres how it works, add a div with an id of theid

Example

<div id="theid">..</div>

you can expand this out a bit if you like and assign the get element to a variable like this

var theid = document.getElementById("theid");

then we tell it to replace the innerHTML of it

theid.innerHTML = "the new content";

the content you see above should say "the new content" rather than the three dots as when the page was loaded

you can also add html in there, just have to watch the ' and " are escaped properly with \