High Sierra

Don't use Firefox for this link...

https://itunes.apple.com/app/macos-high-sierra/id1246284741?mt=12

If you get a corrupted message...

Naw, not damaged, it's just that you have an old copy already in Applications...

Trash the Install MacsOS High Sierra.app in Applications then a new copy more recent version can be downloaded.

Once you have the Install macOS High Sierra.app in the Applications folder

then read these instructions on how to create a bootable USB installer.

How to create a bootable installer for macOS – Apple Support

Using Safari, and only Safari, look at How to upgrade to macOS High Sierra. There will be a link (Get macOS High Sierra) in that article that takes you the Mac App Store. If that does not work, you are done.