Hello Bill:

It's true that you can't use the Unpost function but you don't have to correct it manually.

Go to Inquiries | View Batch Status and locate the batch. It will have a Status = "Post Prior Yr.". Click on the "Recover" button to create a new batch based on the original one. Be certain to select "Reverse entries" and the appropriate date to match the original batch. When you post that batch to "Prior Year", you will effectively be reversing the original batch.

Now you can go back to "View Batch Status", point at the original batch and use the Recover button again, this time without reversing the entries. Edit the resulting batch to make the changes you intend and post it to "Prior Year".