Switching back to iOS12.3 from iOS 13 beta 3
If you have ran a backup using iCloud while on iOS 13, and you don't have iTunes, there is unfortunately nothing you can do. If you have iTunes, you could restore from an old backup if you have one, or you can set up as a brand new phone on iOS 12.3 if you download the .ipsw file for your phone model and correct iOS.