To delete a bank holiday, go to the Bank Holidays tab of your Company Settings.
From here, locate the bank holiday you wish to remove and click 'delete' to the far right of the page. You will be asked to confirm that you want to delete it before it is removed.
Once a bank holiday has been deleted, any holidays that were booked on that day will be recalculated.
NB: Note that this is an admin-only task.