Hi,
Easily what you can do is that, just export a invoice from the database with needed fields, that you have to import back in the system as new invoice.
Once you successfully exported an invoice, this download file can be used as template for further import into the system. Just remove the id column from the file.
Also if you need to get name of any field, you can just activate the developer mode and hover the mouse on the field, in the pop up you will get the technical name of the field, which you can add as column name in the sheet.
Here in this case, it will be partner_invoice_id .
Thanks & Regards
Walnut Software Solutions