You will not be able to edit a holiday allowance if it is assigned to a user and they have booked holiday against it.
In this case, you have 2 options:
1. To delete out all the 'booked' holiday for users who have this allowance assigned to them (ensure you check for ex employees too) and then edit the allowance.
2. Create and assign a new holiday allowance. It is then recommended that you then make a note on the employee profile to say that you have moved them from one allowance to another.
The 2nd option is preferable as the system will not record any amendments made to a holiday allowance, making it difficult to investigate any queries retrospectively.