If you want to quit a chat group, tap the menu button on the top right corner of the chat page. Than tap ‘Leave’ to leave the group.

To delete chats won’t delete the group. On the list, slide the chat you want to delete to the left to view delete option (P1). After you delete this chat, it won’t appear on the list. But if new conversations pop up, you’ll see it again on the list.

(P1)