Add version tag to JS/CSS files in HTML header.
Before:
<link rel="stylesheet" type="text/css" href="https://www.demo.de/js/example.css" />
After:
<link rel="stylesheet" type="text/css" href="https://www.demo.de/js/example.css?v2" />
Optimal: Use a setup script in your project module. (Path: design/head/meta_version_tag)
Manually: Backend > System > Configuration > Design > HTML Head > "Version Tag for JS/CSS Files in Header"
Check rewrites for page/html_head