If this happens continuously, delete the downloaded update, then do a full hard restart by holding down the power and volume down button until the phone shuts off and keep holding the power button until you see the apple logo again, this will do a full hard restart. Then if I were you I’d go either on a windows machine with iTunes, or Finder on a Mac, and download and update the phone through that. It’ll probably work a lot better that way and you avoid having an issue with the update installing… if something happens during the install it can corrupt the file and basically brick the motherboard… it’ll get stuck in a boot loop. That happened to my ex’s iPhone 12 and my mothers iPhone 13… they both had to go to the Apple Store where they both had to have their display taken off, their battery taken out, and have them transferred to a whole new body and new motherboard since the original motherboard was corrupted from the failed update installation. No restore works for either. Luckily they were both under warranty still and apple replaced them for free but it was still a hassle and they didn’t get a whole new phone, just a new rear glass and body, and new motherboard. They keep your original display and battery (the wearable components). I highly recommend going through a computer to update if you have the ability to.