Find the Region you want to edit. If you click on overlapping Regions on the map you'll be shown the names of all the Regions that overlap at that position.
You can then scroll in the left hand panel to the one you want.
Click the cog icon next to the Region name (this is the only way to select any Region for editing, overlapping or not).
That will bring up the Region details in the right hand panel, and will highlight the region corners for editing on the map. Dragging the corners on the map edits the shape of the Region.
You can finish by closing the right panel with the cross top right (to cancel) or by saving changes (bottom of the right panel).