You can downgrade your current subscription on your profile page on the Manage plan tab:
Once you have specified a new subscription plan, you should be able to see what will be changed once you click the Downgrade button:
The downgrade will be scheduled at the billing cycle end.
Downgrading to one of the previous tiers can also be done via the Pricing page. However, downgrades are not subject of prorated charges and refunds. The downgrade will be scheduled at the end of the billing cycle period.
For instance, you purchased a Grow 55 plan on 3 Sept 2022 and decided to cancel this on the same day. Once you downgrade your plan you will be able to see this on your Profile page (Manage plan tab):