what if you dont have wifi

you can also use a computer. If you don't have a computer, ask a friend who has one if you can use theirs or go to an Apple store and ask to use one of theirs.

Or go to a public place like Starbuck or Mcdonalds and use their WiFi.

Update using your computer -

Update iOS on iPhone - Apple Support

You can use mobile data......IF......your iPhone supports 5G......AND......your carrier is actually delivering a 5G signal.

Otherwise, head down to the coffee place, a nearby restaurant, the library, the Apple Store, etc and log on to the Wi-Fi there.

You also might have a friend who has a Wi-Fi router or a Mac or PC.

That won't work.

I gave you several options to try.