Map updates basically consist of two elements, the map content itself, and a code unique to your VIN that lets you load them into your car. This code keeps people from copying the map files from someone else you has them.
The official way to do updates is that you pay the dealer, they ship you a flash drive with the updated map content and the unique code that lets you load it into your car and your car only. All you do is plug the thumb drive in to your USB port in the center stack and the headunit does the rest.
It is possible to do yourself, though it essentially you end up pirating the map content - about as legal and moral as pirating movies, so depends how you feel about it. I doubt you would get into trouble or anything, but...
If you do it yourself, you need to have and be somewhat familiar with ESYS and have the ENET cable to interface with your car, you need the map files themselves (pretty easy to find on the interwebs), and you need to have a program that can essentially create the code you need to allow your car to take the map updates, oh - and a 64GB+ flash drive. If you don't have ESYS or the cable, you might want to get one and familiarize yourself with it anyway, because you can do all sorts of useful things with it (look up "coding" on these forums for all the threads discussing it). It's not for the faint of heart, but it's doable with some time and patience. (start here once you have ESYS: http://www.2addicts.com/forums/showthread.php?t=1054704
, this is a starting point, not a end-all comprehensive tutorial)
You can also purchase this code from 3rd parties that are essentially doing all work above for you. They'll typically charge around $60 (which is fair, if you ask me - this stuff takes time and effort either way) and they'll give you everything you need to update. Keep in mind, these vendors are still technically pirating the maps and circumventing the copy protection put in place by the map makers and BMW/MINI - they are just doing all the heavy lifting for you. Again, it's a legal/moral grey area and there are arguments for both sides that I won't get into, but I think it's at least worth knowing.