Log in to your account and click on 'transactions'. Once there, search for the correct transaction and then just click 'cancel transaction'. Please note that this is only possible if the offer has not been confirmed.

This is instant. No further action is needed. You will not receive payment for a cancelled sale. 

If a transaction has already been confirmed or the cancel button is not there, please refer to this article.

If relevant, please make sure goods and services are returned by the customer, or the customer pays you by alternative means.