PT3 does not delete your hand histories - by default they are moved to the Processed folder (C:\Program Files\PokerTracker 3\Processed) and you can find all of your original hand histories there. If you uninstalled PT3 those files would remain there so that you could use them if need be.
Reinstalling PT3 is for the most part not necessary though. If you wanted to start a new clean database you can create a new database via the Database --> Database Management window by clicking the New button there. You can then set it to active and default to have it be the database currently in use and the one set to be used by default when PT3 starts up. In the new empty database, there will be no historical stats (you could then reimport your hands there if you wanted, or not, however you prefer).
