In most cases, you won't be able to cancel or change a transfer once it has been sent. In fact, according to the detailed rules of the CDS, Canada's clearinghouse for electronic money transfers, once your bank has effectively transferred funds to the CDS via the Large-Value Transfer System, the money cannot be revoked. This allows the CDS to settle the transfer without interference so that the receiving bank can accept it and pass it on to the recipient.
Some transfer service providers and banks may have a small window of time before funds have actually been claimed by the CDS to settle the transfer, during which you can cancel or change your request. It's important to check the policy before you make your transfer. Be sure to hang on to any receipts or confirmation documents/emails you receive for the transfer in case you need to reference this information while attempting to cancel.