once you download it on the PC, unzip it and move the whole folder mapdata_XXXX into '\map' directory on your memory card.

it works for either internal or external on N800 (not sure if you can move it from the internal on N810 to the external, search here for answers)

the last map you used in the application is stored under "mapdata" folder if its the older one you can delete it or rename the directory.

basically you can have all the different maps in directories like
"mapdata_Singapore"
"mapdata_XYZ"
but once you select it in Map application, it will re-name that directory to just "mapdata"