You can cancel your request any time before the cut-off time (listed on Transfer Funds setup screen).

Go to Transfer & Pay, External Funds Transfer and select the Cancel link next to the transaction on the Overview page under Activity and follow the simple instructions. Once your Funds Transfer request has been transmitted to ACH, you cannot cancel the transaction and the Action column will be blank. To transfer funds back to the original account, setup a second transfer between the same two accounts sending the funds back to their point of origin (in effect reversing your first transfer).