You will need to use a tool such as Salesforce's Data Loader or 3rd parties such as Xappex to update all Payment Txns where the field AAkPay__Direct_Debit_Batch__c on the Payment Txn matches the record Id for the Direct Debit Batch.
This process is for experienced Salesforce Admins. If you need more guidance or help, then please speak with your Salesforce consulting partner or you will require Payments2Us Premium Support.