If you're looking to remove a membership payment plan, change the billing day, first payment settings, or minimum contract length on a membership, you would first need to mark the existing plan as not for sale and then create a new payment plan if required.
Check out the below video for steps.
Note. You will still be able to see the old payment plan after you've set it to "no, this payment plan cannot be selected", but customers won't see this from their side. Existing customers will remain on this plan but new customers won't be able to purchase it.