Help! Updating troubleshoot

The model and year of your Mac will determine which macOS you can upgrade to,

please use Safari to access the following links,

How to upgrade to macOS High Sierra - Apple Support

Before upgrading make a full back up of your Mac.

Check that third party apps you use have updates available for the new OS.

https://roaringapps.com

Check that your printers/ scanners etc have updated drivers and software available for the new OS.

Connect your mac to your router rather than rely on WiFi.

Do not use your mac while it is downloading

and do not use any other devices that share the same internet connection.

If when you open the App Store High Sierra page it should say 'Get' if it says 'Open' then the App Store has detected a copy of High Sierra. Open your Applications folder look for Install macOS High Sierra.app, move it to the Trash, empty the Trash.

Restart your mac and try again.

I also have an early 2001 MacBook Pro, in which my hard drive crashed last week. I had already downloaded the High Sierra app, but had decided not to upgrade because it would have made some of my critical apps fail to launch (VMWare for example). I decided when rebuilding that I should go ahead and upgrade to High Sierra, and received the same error as the original poster (File is corrupt). In searching out others who have recently encountered this error, I ran across this post, and wanted to contribute to what finally worked for me.

The above post is correct, if you follow the link. A few key things to mention that are important:

1. Use Safari to access the above link on how to upgrade High Sierra. Can't stress that enough. Other browsers might take you elsewhere, like the iTunes store....where it doesn't exist.....Not that I did that.....and also with every other browser I had installed with the same result....
2. As in the post above, if it says "Open", you already have it, however, it doesn't mean you are on that version. In my case, I was on Sierra, but I had already downloaded High Sierra and it was in my Applications folder, therefore the "Open" prompt in the app store.
3. If you run across the issue that both the original poster and I had that says your copy is corrupt, simply delete it from your Applications Folder *and* empty trash. Go back to the app store, and the "Open" prompt should now say "Download".

Hope this is helpful to some poor soul that ran across the same issues we have!