I want to try Assasin's Creed Valhalla on Stadia but if i dont like the service can i still play Assasin's creed with my PC on uplay or i have to rebuy the game on Uplay/epic to play it with my pc ?
You will need to rebuy valhalla on uplay to play on your pc.
On stadia as long as you do not reach 2 hours game play (or 2 weeks owning the game whichever is first) you can refund it