You can delete a payment by selecting 'Delete payment' in the action menu (three dots) on the payment list row.
If you want to delete multiple payments at once, you can tick the checkboxes next to the relevant payments on the payments list and then select 'Delete payments' (the trash can icon) in the toolbar at the top of the list.
If you are looking to delete all payments in a file, select 'Delete file' in the action menu (three dots) on the file list row.