Can you update your iOS from 11.4 to 15

That would depend on the device. See the list at the bottom of this page:

iOS 15 - Apple

kb1951 wrote:

It is my understanding that you can’t do such a major jump over a WiFi network. Am I wrong on this?

As far as I understand it, iOS 15 is whatever size it is. It's not a bigger file just because the version of iOS currently on the phone is older.

That being said, the OP definitely has a phone too old to do it over cellular.

If your phone is compatible with iOS 15, you can but you will need to use a computer to a massive update like that.

kb1951 wrote:

but you will need to use a computer to a massive update like that.

Really?

It is my understanding that you can’t do such a major jump over a WiFi network. Am I wrong on this?

Thanks for that explanation. I will change my ideas about updates.

kb1951 wrote:

Thanks for that explanation. I will change my ideas about updates.

There is a non-zero chance that I'm wrong..... ;-)