You can change a tax code at any time in the tax year.
Select Employee from the tasking zone and click on View /Edit Employee details. You will see a list of all your current employees, click on the View/Edit link relating to the employee concerned. Now click on the View / Edit button in the Tax Code Details section. If you want the tax code to take immediate effect from the next pay run onwards then click on Edit to change the tax code. Enter the new tax code details and click on the Save button to save the changes you have made. If you have received a notice from HMRC to change the tax code and if the effective start date is much later than the next pay run, click on Add New Tax Code to add the new tax code and enter the effective start date the date the tax code will be effective.