Mobile:
Go to Messages tab
Select Group Message to change name
Click on group message name at top of screen
Click 3 dots (⋮) in top right corner
Click View Team Profile
Click Change conversation name or picture
Type new Name of Group Message
Click Save in top right corner
Web:
Go to Messages tab
Select Group Message to change name
Click 3 dots (⋮) in top right corner
Click Change conversation name or picture
Type new Name of Group Message
Click Save