Downloaded maps are stored in your device memory for use offline when an internet connection is not available.
If you have a current one-year subscription within your app, you can delete mapping detail for areas you are no longer using to free up space on your device. In order to do this simply go to Menu > Delete Maps. You will receive a selection box as you would for downloading maps, which you can adjust before tapping delete to remove the charts. You will be asked to confirm the delete before it finishes.