Go into the Hub and locate the specific plan. Select full or partial refund (specify the refund amount), and submit. The refund amount will immediately be processed and cannot be reversed.
Most Merchants have been set to "refund from future installments equally," which means any refund will reduce the outstanding value of the plan first, before refunding charges back to the customer's credit card. This is the preferred refund process.
Note: there is no longer an option to cancel an existing installment plan. If you wish to close out a plan, you should issue a full or partial refund in accordance with the owed refund amount.