If you get a corrupted message...
Naw, not damaged, it's just that you have an old copy already in Applications, there are 2 cures...
If you’re short of bandwidth or data… Disconnect from the Internet & set the Date back to 2014, then you can run the Install MacsOS High Sierra App in Applications, so expired Certificates are not expired.
Or...As dialabrain says trash the Install MacsOS High Sierra.app in Applications, empty trash & restart, then a new copy of a more recent version can be downloaded…
Don't use FireFox for this link, it’ll offer strange erroneous errors…
https://itunes.apple.com/us/app/macos-mojave/id1398502828?mt=12