I had this issue with iPhone 7.
Did a chat with apple representative and they had me go to my iTunes and make sure I was updated first. Then turn phone off and connect to computer. Then open iTunes and let it install update.
It didn’t work.
She next had hold down the power and volume up button for about a minute or two until your phone shows that it is connected to iTunes, then the iTunes screen on my laptop was showing that it was extracting, then when it was done it started to update.
Phone turned off after installing and I was able to set my phone up.