I assume you are looking at Mapsource on you PC - in which case just click on the maps tab on the left and then click on the maps you want to delete - then hit the delete key - they're gone.
Once done upload to new map set to your GPS unit and in doing so you will erase the existing larger map set.
What do you want to save the memory for ? If the maps are installed then why not just leave them ? Deleting them won't change the speed of you GPS or imrpove anything - you'll just have some unused memory available.