While trying to install El Capitan on my mac Mini 2012 it reports the app may have been tampered with and to delete it and download again. How do I do that? Help

Find the installer app in the Apps folder and drag it to the Trash. It should be named something like Install OS X...

Then go to the Purchased pane of the Mac App Store and download it again.

Late 2012 Mac mini, OS X El Capitan 10.11; Apple Watch, 38 mm silver AL, Watch OS 2; iPad Air 2 & iPhone 6+, iOS 9.0.2

Check that the date shown by the system clock is correct, including the year.