How do I move to a cheaper plan?

Open Change your plan in Billing and choose Move to Launch (or Growth). The move happens at the end of the period you've paid for, so nothing is charged or refunded today.

Before you confirm, Billing lists what switches off on that date and what stays. Your current plan keeps working until then, and the Keep button in Billing (Keep Pro, or Keep Growth) cancels the move any time before it happens.