Instructions cover iOS and Android devices.
The instructions here are for naming or renaming an iMessage group chat.
Open an iMessage group conversation, then tap the top of the conversation.

Tap theInformationicon at the upper-right corner of the screen.
Enter a group chat name.
you’ve got the option to only name group iMessages, not MMS or SMS group messages.

If there’s an Android user in your group, participants won’t be able to change the name.
The group chat name will be seen at the top of the conversation.
Tapthree dots>Group details.
TapGroup name, then enter the new name.
You’re the only one who will see the new name.
Choose a name that’s memorable and meaningful.
Humorous group chat names, where appropriate, work well.
If an Android user is among the group members, you won’t be able to name the group.
you might name group iMessages onlynot group MMSs.