I know there are already a few questions like mine but I wasn't able to fix my problem with their solutions.
I got this code in an external html file to include it in others files:
<div class="header">
<h1 class="replace-me">I'am a placeholder</h1>
</div>
The "file" above gets included into this "file":
<h2 style="display: none" class="variable">Hey, I'm a variable</h2>
The Variable from the second code should replace the content of the h1 tag in the first code. I know this isn't a forum to get free codes, I just want some inspirations or suggestions. Thanks Guys!