This is because the dates, times and leave type were specifically approved by your department manager/ approver. If you want to amend them then only way is to cancel the existing booking and create a new one.
The new booking will follow the normal approval process.
Edit reason or add attachment
If you just want to edit the reason field or add an attachment, click the booking to bring up the full details.
Click 'Edit leave reason / attachment':
Edit the reason and/or add an attachment as you normally would:
Click 'Update request'.