A mandate created by a merchant cannot be modified in the slice application. Contact your merchant in case of discrepancies with the mandate created - this is why it is important to understand the details of a mandate like amount, debit frequency before approving the mandate.
Note: At any point of time, you can pause or cancel a mandate to stop the transactions. If you stop payments while receiving services from a merchant, you might be subject to legal consequences. slice would not be party to any of this.