In order to unlock the form, you need to be a "Reviewer", because "Accountant" cannot unlock the workpapers.

Below example shows that the workpaper is currently in "Completed" status. 

User must  "Approve" the workpaper, then set to "Finished" 

Once the workpaper was set to "Finished", user must "Unlock" the form.    

After unlocking the workpaper, it will now be in "Draft" mode and ready for any edit or changes.