You can update a staff member's holiday allowance at any time. To do this, open the settings tab on the employee's profile.
Scroll down to <name>'s admin settings and enter the new values for their holiday allowance and maximum holiday carryover.
When you scroll back up to the top of their profile, you will notice that their holiday information will have been updated accordingly.
NB: Updating the holiday allowance for an employee will simply change the full year's allocation. As a result, their remaining balance will also be updated. If the employee started with you part way through the current holiday period, a pro-rata calculation will occur to ensure that their allowance is correct. Note that this will pro-rata their allowance based on the new holiday allowance for the whole year. If they had one amount for part of the year and then their allowance increased or decreased, you will need to calculate their pro-rata allowance manually and enter this yourself.