Direct downgrades are not supported.
If you are subscribed to the Plus plan and wish to switch to the Basic plan, you need to first cancel the renewal of your Plus plan. Then, you can re-subscribe to the Basic plan in the next billing cycle.
Here are the steps to cancel subscription:
Note
After canceling, your original plan will remain active until the end of the current billing cycle. Only after the billing cycle ends can you re-subscribe to the new plan.