If you've not bought a PocketPC or Smartphone equipped with GPS, but have a TomTom/NavMan/Garmin all in one "sealed box" type unit - then you can't replace the maps (except for uploading additonal countries or road map updates).
Some Aviation/Marine units also include road maps for use on the ground, but it doesn't work the other way - Joe public booting around the M25 in his or her jalopy does'nt want to swap different maps in/out - they just want directions.
Which device have you got? But I suspect it'll be a "nope".