If it is in pending approval status, Go to Process pay > Payrun progress > select the month/year
Kindly wait for the data to load, and do not click 'show payroll listing report' for it will show a separate report which will not allow you to delete the payruns.
Click on the book icon.
Tick the users you want to reject and then click on the reject button.
If it has been approved, Go to Process pay --> payrun progress --> select the month/year
Click in the book icon. Click on the delete button.