How do I update my phone without internet connection

You may be able to use Ethernet connection from a computer,

and could work. If that model has limited USB ports, etc. ~ Or,

use these other connections, when Wi-Fi is slow; can't work.

Update your iPhone, iPad, or iPod touch with the Finder - Apple Support

//support.apple.com/en-us/HT212185

"If you can't update wirelessly on your device, you can update 

manually using the Finder on a computer that you Trust."

For the most part, nearly any means to update/upgrade systems software

might ultimately be from the Internet; be that by Cable, or Wireless; just the

connective technologies can differ, to include additional software to run it.

Update iOS on iPhone - Apple Support

//support.apple.com/guide/iphone/update-ios-iph3e504502/ios

Update manually or automatically; sometimes little choice exists.

When you need an update a Device to a Latest Version, that

depends on the 'build year' vintage of the device in question.

How to update to iOS 16 or iPadOS 16 - Official Apple Support

As you may be aware, head of updates beyond grasp of old iOS

hardware, these often are spelled out before their public release.

So when that's the case, different paths might then be required.