If you discovered that the IRAS tax export data for some employees are incorrect, the most likely reason is that the pay elements assigned to the employees were not linked to the correct Tax Code in the pay element set up.
If you have already processed the payroll, you can adjust the tax income under the Payroll Setup > IRAS Opening Balance page and enter a value under the opening balance. Negative values are allowed. This will then add/deduct the amount you placed to the amounts generated in the system.
After you have made the adjustments, you need to re-generate the export files.