Question Reset macOS firmware on mid-2010 Mac Pro Desktop ?

[Dustinspcrepair]

High Sierra update failed after firmware update. The drive crashed or OS crashed so I decided to wipe the drive and format El Capitan again. Now El Capitan won’t install because I believe the firmware update for High Sierra still exists. I’m trying to figure out how to reset or change the firmware so El Capitan will install again. Where are firmware updates stored? How do I reset or change it? Is there a way to tell what the current firmware version is from terminal on internet recovery or bootable media? This is a customers Mac so responses are appreciated. I’m at a loss here with absolutely nothing working. Thanks in advance!