It really helps if you can give us more details about what TomTom you own and how up to date it's software is, and also WHAT you are updating.
Many older models suffered from this, but the latest software is supposed to have fixed it.
Having said that, it can still go wrong sometimes.
But there is a very simple solution.....
Firstly make sure you make your backups YOURSELF using Windows Explorer (or Mac Finder) rather than using TomTom Home. Instructions are in our "Articles" section.
Secondly, in that backup will be a file called "MapSettings.cfg" or "mapsettings.cfg". You'll find it inside the specific folder for the map you are using ("Western Europe" North America" or whatever).
Once you have done the update, you should be able to copy that file back into the map folder on the TomTom and it will restore all your Favourites, recent destinations etc.
Usually that works straight away, but occasionally it has been known to cause a re-booting problem, but there's a simple fix for that too.