The "Adjustment" option (on the Documents context menu) to edit the document header of a posted document is not implemented in osFinancials5. 

Unposted documents may be edited before a document is posted to the ledger.  

Once a document is posted (updated) to the ledger, the document cannot be edited. You may access a posted document by double clicking on it (Documents selection screen).

For example; it may happen sometimes that a debtor (customer / client), relocates and still have outstanding invoices with the incorrect (old) address.