To update your iPhone to iOS 16, Go to
Settings > General > Software update > Install Now
STEP 1:
To check whether 5G is available on your iPhone, Go to
Settings > Cellular > Cellular Data Options > Voice & Data
STEP 2:
Two/Three options will be available. Choose between 5G on & 5G Auto. 4G LTE might also be an option.
STEP 3:
Make sure you are using Jio or Airtel as service provider because only these 2 offer 5G services. Also you need to be in a 5G available area to avail 5G services from Jio/Airtel.