Do you have an active Pro Subscription and you want to make it Standard? That is sad to hear but this article is what you need.
If you want to downgrade your current one:
Go to 'Profile' tab - subscription tab would be right below your avatar place,
Selection Subscription of the lower tier,
Complete the purchase with your paywall.
You will receive a corresponding notification if the purchase is successfull
You are Standard now!
The billing amount withh be changed and the upgrade to be applied in the end of the current billing period. You will be charged with the new amount when the new Subscription is applied.