You’re only able to change the duration (the payment period) of your installment plan if the plans have not started (that is, the monthly installment payment has not yet been charged to your account and does not yet appear on your statement).

Otherwise, you cannot make any other changes. However, you may cancel at any time.

See How do I cancel my Scotia SelectPay Installment Plan? for information on how to cancel a plan and what happens when you do so.