High Sierra Update
My current version is Sierra 10.12.6.
When I try to download an Update via Appstore - High Sierra 10.13.6, it's not available - says, that the file is corrupted.
How to download and update my laptop?
MacBook Air 13″, macOS 10.12
This error message usually means that there is a copy of the Install macOS High Sierra.app in your Application folder.
Locate it and move it to the Trash, empty the Trash.. Connect your mac to your router via cable.
Restart your mac and go back to the App Store High Sierra download page, it should now say Get or Download.
Do not use the mac while it is downloading, and do not use any other devices that share the same internet connection whilst the mac is downloading.