You can use Garmin Map Updater. I have seen a couple of way to trick Garmin Express into loading the map to your computer but they are more trouble that it is worth. It is easy to use Garmin Map Updater. Make sure your device is connected. When you see view options select it and select where you want the map loaded.
Here is the link to download. Save it to a location you can find again for new maps.
https://support.garmin.com/support/s...00000000000%7D