Payment Plans are static and cannot be edited as a whole, as they are set agreements with the debtor. Certain aspects of a payment plan can be edited, such as individual payment dates and amounts. This can be done by clicking the green edit pencil next to the appropriate scheduled payment.
If you are needing to change the frequency, amount or payment plan start date, you will need to delete the plan and recreate. All payments that were previously applied to the payment plan before deletion, will still apply to the payment plan when it is recreated, as long as the specific payments are after the payment plan start date.