There is not built in way to delete maps...yet. For now the easiest way is to use the Map Info menu under the Maps menu. Choose Reveal in Finder for the map you want to delete. You can just delete the .gmap bundle to delete the file. To prevent a crash, you should quit BaseCamp before actually deleting the map.
11 years later: this is still the way to do it. You would expect Garmin Express to UPDATE your map if it's indicating it's going to UPDATE your map, and not to keep adding 6.5 GB files to your hard disk. Wouldn't you agree that about 99 % of the people just want to replace their old map? What about a simple extra button next to "Reveal In Finder" that says "Delete Map", and a few extra lines of code that do just that. To innovative?
