The transaction description is pulling in from the account suffix description and then uses the origin code and origin type to determine if it was a payment (i.e. ACH or AFT).  Those are combined to create the transaction description that appears in the member's history.  For example, if your loan product is "360 MORTGAGE" and a payment is made, the transaction description might be "360 MORTG PAYMENT."

You can use Tool #105 "Account Suffix Configuration," choose a suffix and update the transaction description for that particular suffix.  This will make the change for all members with this suffix.