Iphone keeps resetting after softver upgrade

iPhone keeps restarting, spinning gear

Give these steps a try

If your iOS or iPadOS device unexpectedly restarts or displays the Apple logo or a spinning gear icon – Apple Support (UK)

If that does not offer any help then

First try a Forced restart

Force restart iPhone – Apple Support (UK)

If no better

Backup, restore to factory,  and   test, then restore from backup:- 

(If the phone needs attention you will have to do this anyway)

If you can import your media to Photos or iTunes in a computer and copy important documents to a cloud server it will give you some resilience in case the restore from backup fails, or you need to ditch the backup and start as new. 

(1)          How to back up your iPhone, iPad, and iPod touch - Apple Support

(2)          Restore your iPhone, iPad, or iPod to factory settings - Apple Support

(3)          Restore your iPhone, iPad, or iPod touch from a backup - Apple Support

If the problem remains then do step (2) again but set up as new without restoring any backup.

 If it fixes it then it is probably a rogue app.

Otherwise arrange a repair

iPhone Service & Repair - Apple Support