We don't recommend the deletion of actual conversions - we will always advise to disapprove conversions instead.
However, in certain cases (such as test conversions that were tracked), the ability to delete conversions can be quite handy.
To delete a conversion go to your Conversions page, then click on the issued conversion. The right side panel will appear. Click on the three dots on the upper-left side of the panel. The option to Delete conversion will appear.
If you'd like to delete a large amount of conversions, you can do so via the API.
P.S. If you are looking for a solution to manage refunds/cancelations you can refer to this guide: How to handle refunds/cancelations.