Overview
Why would you do that? No idea. But should you want to…
Preparation
First, turn off SteamCloud.
To do that, rightclick the game in Steam and disable Steam cloud for this very game.
Then, navigate to shell:PersonalGiana Sisters – Twisted Dreams
Yeah, the path looks weird, but you can copypaste it into the Run box.
Once there, delete Progress.dat
Finalization
Before doing anything else, start the game and complete one level.
Now you can turn SteamCloud back on and have fun with a fresh profile!