To adjust a staff member’s holiday allowance, you will need to go to their ‘Staff File’:

Under the ‘Common Options’ tab of the full admin, there is ‘Add / Edit Staff Records’, click this and then find the appropriate staff member.

Once in the desired staff member’s file, go to the ‘Holidays’ tab. 

You will then just need to click ‘Adjust Carry Over’ and another small window will appear. 

To add hours, put a ‘+’ (plus symbol) and then the number of hours you wish to add. 
To subtract hours, put a ‘-‘ (minus symbol) and then the number of hours you wish to deduct. 
I would recommend adding a reason for your reference later on.

Once that information is complete, press ‘Process…’ and the hours will adjust accordingly to the options you have chosen.