Go to the Vouchers service.
Select Search Sold Vouchers.
In the Search Sold Vouchers screen, search for a voucher by reference number, internal reference number or customer name.
Select one option and click Search Vouchers.
Click the Details button.
Click edit on the voucher
Change the balance on the voucher to what is desired.
Click Save Changes.
Note: Changing the balance amount also writes a log into the voucher comments.