How to do the software update

To update to the most recent, compatible version of iOS 15.7.3, please see: Update your iPhone, iPad, or iPod touch - Apple Support

And: If your iPhone, iPad, or iPod touch won't update - Apple Support

If the above Apple articles do not help, please post back with relevant information. 

Go to your SETTINGS .. click on GENERAL and look at the top for SOFTWARE UPDATE

Go to your SETTINGS .. click on GENERAL and look at the top for SOFTWARE UPDATE