The answer to this is much the same as for how you installed the map in the first place, except you are removing/deleting the map instead.
Simply either insert the MicroSD card into your computer (if you used a MicroSD card) or attach your Garmin device (either with or without a MicroSD card) to your computer (Mac, Windows, Linux or Chromebook) and go to the GARMIN folder in the relevant file manager on your computer. From there simply highlight the relevant map file (mine usually have TT50 or TTSTD in the filename) and then delete it.
Remember to empty your Trashcan or Recycle Bin afterwards (as the file is not really deleted; it is just marked as such on the device filesystem).
If you have a Mac and a Garmin device that uses MTP USB mode (such as the latest Edge models, Fenix, Epix, Forerunner, Enduro, etc.), you will probably need to use a third party tool, such as OpenMTP, AndroidFileTransfer, CommanderOne, or similar to see the Garmin as a mountable (browsable) filesystem.