Hi Rognik39,
You're correct that once a pay run has been lodged you will see a message stating you cannot unlock it.
You can create an ad hoc pay run for the same period to make the amendments, e.g. adjust PAYG or reverse figures that are incorrect.
Once the pay run is finalised you will then be able to create an Update Event for STP. Please see here for more information.
Please let me know if you have any other questions at all.
-Rebecca
