What i did (an this may not be the right way or the easiest) is connect your tomtom to the computer, copy all the files on your tomtom to somewhere safe on the pc.
Once youve done this use tomtom home to ensure your navcore is upto date. After that make sure everything works an then delete the files you made a backup of earlier from your pc.
Now make another backup by copy an pasting again so you now have a working upto date (except map) copy of tomtom on your pc should anything go wrong.
Goto tomtom home again and click update map it should give you the map number which is recommended for your tomtom, for example for me its 845.2666.
Now you need to find that map on here or any other site you like and download it.
Once you have the correct map delete the old one from your tomtom device (dont worry remember its still backed up on the pc) and copy over the new map you just downloaded to your tomtom.
People go on about different keygens and ways of patching, however I always use this program and have never had any problems.
Its easy to use and very self explanatory but you can find any help you need using the keygen on the first page anyway.
sorry no links to other forums
Hope this helps.