Employees cannot delete or cancel their own leave. Only authorised users (HR or managers) can do this. If leave has been entered incorrectly, ask your HR manager.
Employees can enter leave for past dates; the restriction applies only to deleting and cancelling.
If you are an authorised user, delete leave like this:
Go to Leave > Leaves in the left-hand menu.
Tick All Leaves at the top of the page.
Click the delete button to the right of the leave you want to remove.
Approved leave can be deleted the same way.