If you have finalised the pay but not yet paid your staff then you can reverse the erroneous pay for the employee and run another pay for just that employee.
If the payment has been made there are a couple of options. The quick and easy option is of course just to edit their next pay, reducing they amount the are paid. The disadvantage of this is that it doesn't provide an accurate record of why the pay differs.
We recommend creating a new pay code called something like 'Overpayment' or 'Pay Adjustment'. It should be of the same 'Tax Type' as the overpayment amount - if you overpaid their normal earnings then choose 'Gross Earnings'. If you overpaid them a Bonus it should be a 'Taxable Allowance'. All the other settings on the pay code should be the same as the pay code you overpaid on.
Then just add this pay code into the employee's next pay, entering as a negative amount the overpayment.
The advantage of this is that the payslip will now show a separate line for the adjustment, labelled appropriately.