how do i name a group text

As it says in the Related Article at the top of this page that you were reading when you posted, As well as the (same) link provided by ResortCasual, you cannot name a group text if there are any SMS (as opposed to iMessage) users in the group. It’s a limitation of the SMS protocol. You also can’t remove members from any group text that includes non-iPhone SMS users, something to keep in mind when creating a group message.

This support article from Apple should help you rename a group text message on your iOS devices.

https://support.apple.com/en-us/HT211301