You use a program called TomTom home.
Worth plugging it in and doing software updates/ firmware updates, even if you don't want to pay to upgrade the maps. Irons out bugs.
Unfortunately on all sat nav's you have to pay to update the maps.
TomTom have got a Mapshare thing, which uses update themselves, and you can load all this onto your satnav via the same program, which should hopefully update it. Though still won't show new roads obviously.