I need to adjust the payments of Nic for an employee who reached state pension age in 09/20 but the adjustment to catergory C was not made until december 21

Hi val32, any rebate due will be calculated once the payroll has been rolled-back and re-submitted with the correct tax code applied - HMRC carry out an automatic reconciliation for PAYE employees at the end of each tax year for tax paid throughout the year and notify or any credit due for overpayment on the p800. As the change applies to a previous tax year, it would not be possible to roll back and we would recommend contacting HMRC to see if they are able to offer an alternate solution. 

Hi @val32 

As this goes over two tax years, I think the only way to sort this out will be to contact HMRC & get them to refund the employee.

Or get their authorisation to refund the employee & offset against your monthly NIC/PAYE payments - as, technically, it's your mistake not the employee's or HMRC's.

NIC is not like PAYE - it's non-cumulative - so is very difficult to fix errors without causing other problems.

Was the employee's DOB entered incorrectly?

This really should have been done automatically by QB Payroll - or at least flagged by HMRC RTI.