This article relates to Transactor 6
This is possible using the Tools > Utilities menu, then using the 'Un-Lock all locked documents' button. After that has run (which can take over an hour depending on the amount of locked data you have!), you can do what you want, then use the Lock Finalised Documents menu item to re-lock them up to the previous month.
Note that once you modify old jobs you will have to allocate/unapply all payments on that account, then allocate/auto-apply from the oldest first, or you may end up with unallocated credits!