I've had a good search on the forum and massive thanks to jack9110t who posted step by step instructions of how to do it here:
http://www.navitotal.com/maps-for-navigon-f93/navigon-europe-2015-fresh-maps-mn7-mn8-t18070.html#p146953
Just in case the link doesn't work, here's the guide - I've just done it and it's worked for me
"Hi,
since maps + more has not got enough memory for newest maps it is necessary to remove some files from the navigon in order to update correctly.
You have to follow those steps if you want to update your navigator:
1- Connect your Maps + more to a Windows computer, since Mac are non supported unfortunately by this device, and browse to NAVIGON folder
2- In NAVIGON folder go to map and delete all the files in it but not the folder
3- Go back to navigon folder and if you have a VW delete the folder skin_SEAT and skin_SKODA otherwise delete all the folders that are not those of your car brand
4- Remain in NAVIGON folder and open sounds folder: here you can delete all the .nfs files that are not those of your language. Be careful it is very importante to not delete .bin files in that folder otherwise you will no more able to use navigation
5- Go back to map folder and copy all the files that you want from the recents that you have downloaded. in particular you have to copy the maps .map and all the data files you found in the download where the extensions are .nfs
So copy .map (that you find in map folder) and all the .nfs (that you find in data folder) in NAVIGON/map
Now you can enjoy your updated maps+more
"