Try via iTunes. If that fails make sure you have a backup, then put it in recovery mode and restore via iTunes, then restore from your backup.
I had the worst experience upgrading my iPhone 8 yesterday from 12.4.1 to 13.1. It put it into a mode that would never finish. Multiple forced reboots, no go. Tried updating while in recovery mode, also no go. Had to restore the phone and then download backup. Took pretty much all day trying to get it resolved.