Contact the service provider who issued the prepaid debit card. The phone number is commonly printed on the back of the card. If you purchased the card at a bank or store, keep in mind that these sellers usually aren't the parties who can dispute charges on your behalf for a prepaid card.
Explain the circumstances surrounding your reason for disputing the charge. In some cases, like an obvious duplicate charge, the provider may be able to delete one pending transaction. In other cases, such as an unauthorized transaction, the provider may tell you to contact the merchant to get the pending charge reversed through a refund.
Allow the pending charge time to post to the debit card account if you cannot have the amount removed while it is pending. Call the provider again to issue a dispute after it posts and await a decision. If the provider agrees to reverse the charge you either receive the amount as a credit back to the prepaid card or as a check in the mail.