I want my page to have a navigation menu, but I don't want to copy and paste it to every webpage every time I update it, so I'm trying to put it in a separate file and load it into a div
You can use w3-include:
<!DOCTYPE html>
<html>
<script src="https://www.w3schools.com/lib/w3data.js"></script>
<body>
<div w3-include-html="content.html"></div>
<script>
w3IncludeHTML();
</script>
</body>
</html>
Or PHP:
<html>
<body>
<div class="menu">
<?php include 'menu.php';?>
</div>
<h1>Welcome to my home page!</h1>
<p>Some text.</p>
<p>Some more text.</p>
</body>
</html>
There are other ways too, I just included two.