Only Payments Administrators can change or delete Payment Methods. You can do so in the Open Transactions section. To get there, navigate to the Transactions page by clicking the link in the cog in the top right of your screen:
You can now click the “Delete” button in the rightmost column of your active Payment Method. Once you confirm the deletion of this Payment Method it will be permanently deleted.
From here you can add a new Payment Method as described above.