Hmm turning off during an update is highly discouraged. How long has it been? For me an update over cellular takes about 10 minutes.
Just sit it through. If you need to make calls, isn't there a house or friend's phone to borrow?
If the update is 8.1 it'll take a while because it's a pretty sizeable update!
For the record I don't know any phone in the world that likes being turned off mid-update. It can screw things up. Make sure it's plugged in for the duration of the update; you don't want to risk it running dry.