Open the Messages app
Open the group text
Tap the group name or the contact icons at the top
Scroll down
Tap Leave this Conversation
Tap Leave this Conversation again to confirm
If Leave this Conversation is not shown, the group is not an iMessage group or the group has fewer than three people
In that case, mute the conversation by tapping the group name, then Hide Alerts
You can also delete the conversation from your Messages list if you want to remove it from your phone