Over heating
After update the phone will over heating without any abnormal uses
Well, the update procedure requires many resources from iPhone's battery.
Read this support article too:--> If your iPhone or iPad gets too hot or too cold
If you think it's still overheating, for your peace of mind I suggest that you make an appointment (*) and have your device inspected at an Apple Store or AASP (Apple Authorized Service Provider) among this list:--> Apple – Location Selector. After checking it out, they'll give you all the available options.
Regards
Giulio
(*) You can do it:
- Online:--> Genius Bar Reservation and Apple Support Options - Apple;
- Contacting a local Apple Store or AASP among the list provided above;
- Using the "Apple Store" app, available and downloadable from AppStore.