If there is only one employee concerned, it can be done from the employee management in the vacation tab:
Just type in the correct balance.
If there are multiple employees involved, this can be done from the selective update of the employee management :
Do not forget to click on Apply before closing the window otherwise the changes will not be saved.
If you opened the new year before you finished recording the vacations in the old one, you can reset all the balances at once:
Go to Diagnostic:
Select Tools - Employees tools - Recalculate the initail balance according to previous year :