Some people play with the operating system software, and download files to do that. You need computer knowledge.

The map is one single file - a massive database of streets etc - which for europe is something like a single 4GB file - and to update that you have to get a new map disc.

Some people add extra POI information, which can be done to some map discs in some countries (eg: cannot be done in my country) but adding streets and towns is simply not possible.