How do update my phone. The cellular and phone service will not work.

Start here:--> If you see an Update Failed or Apple Pay Not Available alert after iOS update or restart - Apple Support

if the issue persists, contact Apple Support directly, using this link:--> https://getsupport.apple.com/?caller=kbase&SG=SG003&category_id=SC0999&symptom_id=99976

If you mean something different, please specify your needs.

Regards

Giulio

Go to Settings/General/About and look at Modem Firmware. Is the entry blank? If it is you need to reinstall it by restoring the phone to factory settings, then restoring a backup. Back up the phone first, of course:

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 modem firmware DOES have a value you may have a hardware problem. The first step is what I just posted, but after that you need to contact Apple support using the Get Support link at the top of this page.