Your discount codes will be listed in the admin at My Tickets > Discount Codes, and can be edited or deleted from there.
Editing or deleting discount codes will not change past purchase or any accounting references within the plug-in. However, deleting or modifying a code while it’s being used (e.g., between the time a user adds the discount code to their cart and when they check out) can have strange results.
The discount codes table shows you all of your discount codes, what their discount is, and the relevant information about how they are applied.
My Tickets does not track usage of discount codes.
Deleting a Discount Code is instantaneous, with no recovery option for the deleted code.