My iphone is not updating

If your iPhone 7+ is still running iOS 12 (as per your statement), you will have many difficulties updating it wirelessly (via wifi).

I suggest that you try to update it using a computer running iTunes (Windows PC) or Finder (Mac). If you don't have a computer, borrow one from your friends/neighbors, or (much better) ask for help at an Apple Store or Authorized Service Provider among this list:--> Apple – Location Selector

Regards

Giulio