Go to Settings>Data Downloads and tap on the area you’d like to delete. An option box will appear, allowing you to tap delete.

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If you're wishing to delete a map overlay (i.e. kml or kmz files), tap the Map Settings button

Scroll the left column of options to the bottom of the list. Locate the overlay you wish to delete, then swipe it from right-to-left.

A Delete button will be revealed. Tap it and the overlay will be removed from all of your devices.