Software update
How to update software on iPhones 7 that doesn’t show software update option and on also is show no service
iPhone 7
In Settings->General->About->Model what does the iPhone have?
In Settings->General->About->Version what does the iPhone have?
What are you trying to update to exactly?
The "No Service" error means either there's an issue with the inserted SIM card, an issue with the cellular service provider or service plan, or an issue with the cellular hardware in the iPhone.
Make sure there is a SIM card correctly inserted in the iPhone. Make sure it has active service, and make sure you are in an area with good cellular coverage.
See: link-> If you see No Service or Searching on your iPhone or iPad - Apple Support
and: link-> Remove or switch the SIM card in your iPhone or iPad - Apple Support