An interactive counter

JavaScript changes the page after it has loaded, and working with it always comes down to three steps: you select an element, listen for an event on it such as a click, then change something on the page.

document.querySelector('#btn') selects the element, addEventListener('click', ...) listens for clicks, and the textContent property changes the text.

You'll also need a variable that keeps the count from one click to the next, so define it outside the listener function, or it will go back to zero on every click.