To name a new group chat, click New chat at the top of the app. Click the down arrow next to the To field and enter the chat name in the Group name field. This creates a whole new group chat, even if you already have a chat going with the same participants.
To name an existing group chat, click Name group chat next to the list of names of everyone in the chat (at the top of the screen).
You can change your group name anytime by clicking Name group chat again.
To name an existing group chat, tap the chat header and then tap Name group chat.
To name an existing group chat, go to the chat header and tap More options > Chat details > Name group chat.