Mac OS High Sierra is damaged

[FFFalcon]

Hello! I've been trying to upgrade to High Sierra for three days, unfortunately every time I try to install through App Store it says that the install app is damaged, I don't really know what to do, any advice?

 

[CoastalOR]

Check your Applications folder for "Install macOS High Sierra" app, if/when you find it delete it (it has an expired certificate) and download a new replacement Install macOS High Sierra app from the App Store.