Hi ,

As Mohammed Amal told , it is declared as readonly field in the states other than the draft state. To achieve what you are looking for, you may have to edit and remove the readonly attribute from the corresponding  python file in the original code or you have to code a custom module, so that you can redefine the same field without the readonly attribute.

Thank you

In the python file it is declared as readonly for all states other than draft.

So I think you can not edit it once you validate an invoice