A group is deleted automatically when all its members have left. If there are still members in a group, follow the steps below to delete them first.
1. Tap the menu icon at the top of the group chat screen > Members.
2. Tap Edit at the top right of the screen.
3. Tap the Delete button next to the member you want to delete.
4. Tap OK.
After all of the other members have been deleted, you can delete the group by leaving it. For the steps to leave a group, see this Help article.