If you'd like to be able to delete PTO as an Administrator or Manager with approval access, you must first enable "Shifts and PTO can be Deleted" from within your Settings --> Account Settings:
Once that option is enabled, there is one of two ways to delete PTO. The first option would be to delete the PTO directly from the employees time card. There will be a delete button, which looks like a garbage can, to the right of the PTO:
The second option is to use the PTO calendar to delete the PTO. You'll want to select Timecards in the top navigation followed by PTO Calendar:
Once in the PTO Calendar, you can select the Delete button location in the top right and then check the boxes next to the PTO you'd like to delete. Once done, select the Delete button again.
You can also click directly on the PTO and then choose the Delete button on the "Edit PTO" page: