M-Power uses the payment terms specified in Xero for each customer on invoices. When invoices or bills are exported to Xero no payment term information is sent, whatever terms are already set for the customer in Xero is the default for the new invoice/bill when it arrives in Xero.
To modify or update the payment terms first make the required changes in Xero then follow this procedure:
1. Open an invoice for the customer in question. Then check that there is a customer selected in the Xero Link  field. Then double-click the Terms of Trade field 
2. Click Yes in the 'Update' dialog. Wait while the terms are updated. Once complete you can print/email the invoice as required with the updated terms.
Note that payment terms need to be set in Xero before they can be retrieved. Xero does not expose a default term.