Stats never need to be "reset". That is because Stats are generated only from the tournaments you specify, by using the Filter. In the Filter you can specify a date range, or season, or league, or any combination of those, to determine from which tournaments you want to generate your stats.
The easiest thing (I think) to do is for you to now create a Season on the Database tab. Then on the Stats tab create a new Filter and select the appropriate Season. When you start running your tournaments, make sure to select the correct Season on the Game tab. When you run stats using that Filter, you'll only get tournaments from the appropriate season.