If you haven't advanced the payrun, you can re-open the payroll for the period if you find that you have additional entries to make. The journal which was automatically posted when you initially approved the payroll will be reversed at the point of un-approving.
Note: if you have submitted your RTI file, you will need to resubmit so that any changes made are reported to HMRC.
Select an Employee and at the top of the screen selected you will see a message box, as shown in the example below. Click on the remove the approval option and the payroll will be reopened for the period.
Once you have removed the approval, you will see another journal has posted to reverse the initial journal posting. The reversing journal will have the same journal reference and will include (R) at the end.
Now, you should make any changes needed, and then approve the payrun again. You will need to print amended payslips and resend any RTI submissions to HMRC.
Once you have advanced your payroll into the next period, you will not be able to re-open previous pay periods.