Yes, you can edit or cancel a future-dated payment anytime before the send date. You can only edit certain fields, which include Send Date, Amount, Subject Line, Personal Message, and My Notes.
You can also stop a payment anytime after the send date if the payment has not been deposited into the recipient's account. To stop a payment, go to Activity and click the Stop Payment button. You will not see the Stop Payment button if the option is no longer available.