You can find information on Stadia's return and refund policies here, if that's the route you'd like to go:
Or, if you'd like, we could try to help you troubleshoot and resolve the problem(s) you're facing. In that case, it would really help us help you if you could provide some more details. How are the games unplayable - input lag or something else? And what's your setup - PC, Mac, Chromebook, etc.?