Allowance changes on a user's profile are not recommended as is easier to set within Time-Off Policies, but then again there are certain scenarios where it may be best to change the allowance on each specific user.
Steps to update the allowance for each user separately:
Time-off module
Manager users
Edit user settings for the user in question
Allowance tab
Edit for the time-off type you want to adjust the allowance
Enter the new allowance under "Yearly Allowance"
If you want this change to affect the balance by removing or adding the difference simply select "Yes" under the "Modify Balance" option.