I foolishly downloaded some OpenStreetMap maps, and now can't seem to remove them from my GPSMAP. I've even Reset All Settings, and yet the maps remain.
You will have to delete the map files from the Garmin Folder of the device directly by connecting it to a computer via USB. These articles have steps that may help: Removing Maps From an Outdoor Handheld Using Windows or Removing Maps From an Outdoor Handheld Using a Mac.