iOS setup assistant

"Setup Assistant" runs automatically when you power up a new phone for the first time, or if you have erased the phone and it is restarting.

If you have already powered up the phone and changed a few settings......and you want to setup the phone again using "Setup Assistant"........you will have to erase the phone and start over again with a new setup.

Not sure why you would want to do this, but here is how to do this:

Settings > General > Transfer or Reset iPhone > Erase All Content and Settings

Maybe if you could provide more details about what it is that you are trying to do, we could help you avoid having to erase the phone and starting all over again with a new setup.

It's not clear exactly what you're trying to do here? This link may help you --> Turn on and set up iPhone - Apple Support