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

Once in the Time Off Calendar, you can select the Delete button location in the top right and then check the boxes next to the time off you'd like to delete. Once done, select the Delete button again.

You can also click directly on the time off and then choose the Delete button on the "Edit Time Off" page: