Mandates do not have an expiry date. They will remain valid until canceled. This can be done by sending an email to your country's Service Desk support (please refer to Modern Support - Contact Directory). Please be aware:

Canceling your mandate makes pending or future Direct debits against that Mandate void and repayments will need to be done through another payment method.
The Reseller can always reapply for a new mandate, such as in the case of a new bank account.