Cellulare update failed how i can fix my phone to working again

iphone 7

model A1660

memory 128 G

ios 9.1

i update my phone in last week

for 1 week my service is so weak it’s means in 24 hours i have service just for 10 or 12 hours

and today i rest all setting after that I don’t have any service at all and i see that alart cellula Update failed

now i want to know how i can fix my phone to work again

thank you