Before starting, it's crucial to backup your Mac. Use Time Machine or any preferred method to ensure your data is safe.
Standard Apple support pages lead to a loop. Searching "download macOS High Sierra 10.13.6" usually takes you to a broken App Store link. The so-called exclusive source (in this case, a verified developer archive) delivered a installer named InstallMacOSHighSierra.dmg . download+macos+high+sierra+10136+exclusive
Copy and paste the following link into your browser (this is the official, signed Apple CDN URL): Before starting, it's crucial to backup your Mac
If you want, I can: