To get the element in JS by its class name you would do something like this:
document.querySelector('.content')
This would select the element with class “content”. You can change the content of this element in JS like this:
<!-- just text -->
document.querySelector('.content').textContent = "Test";
<!-- HTML -->
document.querySelector('.content').innerHTML = "<h1>Test</h1>";
I hope I understood your question correctly. If not, maybe a link to the exercise (if any) would help.