To leave a subgroup, open the subgroup chat and tap the header to open subgroup info. Scroll to the bottom of subgroup info and tap "Leave this subgroup."
If you want to remain in the subgroup but stop getting notifications for it, choose mute instead. You will remain a member of the subgroup, but will not get push notifications for new messages. It will be removed from your chat list and accessible from the Super Group menu.
Leaving a subgroup does not remove you from the entire Super Group. To leave an entire Super Group, see How do I leave a Super Group?