How to stop update

At what stage in the process is it?

If it hasn't started yet:

Settings > General > iPhone Storage (or iPad Storage)

Select item and Delete Update.

If it is actually to the stage where you see an Apple logo and is installing, it is likely too late. If it is just preparing, you could try shutting down your iPhone but I haven't tried doing that and don't know if that will truly stop it or if it will just start again.