1. Copy the USA_Canada map folder to the computer using Explorer.
2. Once you are **sure** the map folder is safely on the computer (remember where you put it), then delete the USA_Canada folder from the device.
3. Copy your Europe map folder from the computer (I assume it is there) to the unit again using Explorer.
4. When back from Europe, just reverse the process.
OR.....
Method B: (if using the 920, I think the 540 also has a card slot)
1. Get yourself a mini sdhc card for your unit of maybe 8 gb in size, the faster the speed (class 6+) the better.
2. Install the Europe map on the card.
3. When you want to change maps, just (in the device) go to manage maps-->switch maps.