Realistically, I respectfully suggest that you need to upgrade to a Zumo 660 if that's really the case.
As mentioned above the 2820 doesn't have sufficient storage space for the more recent map sets. It's possible to put the majority of Europe, probably all you'll ever need TBH, but it's something of a protracted process. The Garmin updater does offer the option of loading one of a set of pre-defined regions of Europe onto units without the space for the whole lot. However, in my experience, these are not ideal. You can only choose one and ISTR that they bisect Europe in a way that's not ideal for someone in the UK.
Some time ago I asked Garmin to provide a definitive list of what's included in each region. Remember, you can only choose one of these:
- Alps - Austria, Northern Italy, Switzerland, Eastern France (small amount), Southern Liechtenstein (small amount)
- Alps & DACH - Austria, Eastern France (small amount), Germany, Liechtenstein, Northern Italy (small amount), Switzerland
- BeNeLux - Belgium, Luxemburg, Netherlands, Western Germany (small amount)
- BeNeLux & France - Belgium, France, Luxemburg, Netherlands, Western Germany (small amount), Corsica region, Saint Barthelemy, Guadeloupe, Martinique, Reunion, French Gulana
- Central & Eastern Europe - Albania, Austria, Belarus, Bosnia and Herzegowina, BULGARIA, Croatia, Czech Republic, Estonia, Germany, Greece, Hungary, Italy, Latvia, Liechtenstein, Lithuania, Macedonia, Moldova, republic of, Montenegro, Poland, Romania, Serbia, Slovakia, Slovenia, Switzerland, Turkey, Ukraine
- Central Europe - Austria, Belgium, Czech Republic, Denmark, Eastern France, Germany, Italy (about ¾’s down to the top of the boot heel), Liechtenstein, Luxembourg, Monaco, Netherlands, Slovenia, Switzerland
- DACH & CZ - Austria, Czech Republic, Germany, Liechtenstein, Switzerland
- DACH & Eastern Europe - Albania, Austria, BELARUS, Bosnia and Herzegowina, BULGARIA, Croatia, Czech Republic, Estonia, Germany, Greece, Hungary, Italy (about 90% doesn’t cover full southern part), Latvia, Liechtenstein, Lithuania, Macedonia, Moldova, republic of, Montenegro, Poland, Romania, Serbia, Slovakia, Slovenia, Switzerland, Turkey, Ukraine
- Full Europe - Albania, Andorra, Austria, Azores Islands, Belarus, Belgium, Bulgaria, Canary Islands, Channel Islands, Croatia, Czech Republic, Czech Republic, Denmark, Estonia, Finland, France, French Guiana, Germany, Gibraltar, Greece, Greece, Guadeloupe, Hungary, Iceland, Ireland, Isle of Man, Italy, Latvia, Liechtenstein, Lithuania, Luxembourg, Macedonia, Malta, Martinique, Madeira, Moldova, Netherlands, Northern Ireland, Norway, Poland, Portugal, Reunion, Romania, Saint Barthelemy, Serbia & Montenegro, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey, UK, Ukraine, Nordics, Iceland, Norway, Denmark, Sweden, Finland, Northeast Europe, Austria, Belgium, Czech Republic, Denmark, Estonia, Finland, Germany, Hungary, Iceland, LATVIA, Liechtenstein, Lithuania, Luxemburg, Netherlands, Norway, Poland, Slovakia, Sweden, Switzerland, Northwest Eastern Europe, Austria, Croatia, Czech Republic, Hungary, Poland, Slovakia, Slovenia, Northwest Europe, Andorra, Azores, Belgium, Canary Islands, France, French Guiana, Guadeloupe, Ireland, Luxemburg, Madeira, Martinique, Monaco, Netherlands, Portugal, Reunion, Saint Barthelemy, Spain, UK, Southwest Europe, Andorra, Austria, Azores, Canary Island, France, French Gulana, Guadeloupe, Italy, Madeira, Martinique, Monaco, Portugal, Reunion, Saint Barthelemy, Slovenia, Spain, Switzerland, Western Europe, Andorra, Azores, Belgium, Canary Islands, France, French Guiana, Guadeloupe, Ireland, Luxemburg, Madeira, Martinique, Monaco, Netherlands, Portugal, Reunion, Saint Barthelemy, Spain, UK, Switzerland, Italy, Austria, Germany, Liechtenstein, Greece, Denmark, Norway, Sweden, Finland
You should save the update to your PC as well as that will allow you to plan routes on your PC which is where the real strength of the Garmins comes through. Once you've done this you can create your own custom map set with the bits of Europe you need. Unfortunately getting this mapping from your PC to the GPS might well require a bit more mucking around with files both on your PC and on the GPS than you comfortable with. Additionally I think you'll need Garmin Mapsource on your PC before you save the mapping update there, did the 2820 come with any disks? If not then you've got even more mucking around with the PC to do.
Added to which, if not done already, the previous owner will need to re-register the unit with Garmin and you will then need to set-up a myGarmin account and register the 2820 in your name before you can start to obtain updates. Without this recognised change in ownership you'll be stuck with what you have now.
Part of the web update service will look to see if the firmware is current, if it isn't that might improve the routing but I doubt if it will have much effect to be honest. The device is pretty dumb and can only work with the mapping data, which is fairly rudimentary TBH, and it's routing algorithm to calculate a route. As I say above, route planning on the PC is the real advantage and where the Garmins knock spots off the other makes of GPS.
The A- B routing isn't always going to be that logical in the real world. It can look at distance and the road types/speeds in the data and decide a route based on your preference for fastest or shortest route. You set the latter in the menu on the GPS, fastest is usually best as shortest will take you by any old goat track (some here prefer that).