It is not possible to delete or edit a transaction within a BBMS account.

In compliance with regulatory and financial industry rules, clients are unable to delete transactions from the BBMS portal once they have been processed. Not only does this help mitigate the risk of fraud and money laundering, but it is essential for proper reporting within our systems and card brand networks.
 
Blackbaud protects all information against loss or theft as well as unauthorized access, disclosure, copying, use or modification. For additional information regarding security of cardholder data, please visit our webpage on Blackbaud's PCI compliance: https://www.blackbaud.com/security/pci-compliance

Please see How do I refund a transaction if funds need to be refunded to the cardholder.