Note that the existing maps will be 'cleaned up' before the new ones are installed, and that the process takes around five to ten minutes, depending on the country maps that you decide to install/update.
In Nokia Maps, tap on the menu and then 'Map Loader', then on 'Check for updates', with the sequence then as shown below in these screenshots:
Look for 'Map loader' on the Options menu in Nokia Maps, then check for updates using the button provided. The byte count above is for the whole of Great Britain, if you're interested.
Note that this is a totally new map set, so the old maps first get deleted (don't worry, you won't lose your 'favourite' places); the update in progress, it takes around five minutes for most country's roads.
Again, it's great to know that the offline maps on our Symbian phones are as up to date as those on Nokia's very latest Windows Phones (where, curiously, they're rebranded as 'HERE Maps' - go figure!)
Comments welcome if you see an update for installed maps (via Nokia Suite) for older phones, i.e. S60 3rd Edition