Are you wondering if you are able edit or amend an existing allowance?
You are not able to edit an existing holiday allowance that has been used to book leave on. This is because the allowance is now tied to historical leave records.
What you will need to do in this case is either:
Purge all of the holiday data of those assigned to that particular holiday allowance ( This will delete all traces of leave within the system permanently)
Create a new holiday allowance with the new allocation and then assign this to the relevant users.
Below are a couple of links that will be useful to implement the above workarounds: