Go to the tab ShiftPlanning > Leave Schedule and find the right date & employee name whose leaves you are trying to remove.
This will show the dates and names of who is approved and allow you to click on a name to remove an entry.
Note: This article was pulled from the Humanity support system. For the most recent update, visit https://helpcenter.humanity.com/faq-s-and-troubleshooting/shiftplanning/how-do-i-cancel-an-employees-vacation-request-once-already-approved.