As we have a GOBD conform system, we are not allowed to alter vouchers afterwards or delete them completely.
To change the payment terms we need to cancel the invoice with a credit note. Therefore we need to go to sales->records->invoices and search the invoice we want to change. Now there is in the bottom right corner a button which says "create credit note". Click on this button. Now just hit next and save.
On the bottom left corner we are able to change this credit note into an invoice. When you are back in the invoice window, click on "1 Voucher head" alter the payment terms hit next and save.For vouchers created in the cash register, follow the steps below:
1. Put the same article, if available even with serial number, into the cash desk.
2. Click into amount and enter a "-" in front of the amount. confirm twice with the enter button.
3. Choose the WRONG payment term.
4. Put the same article into cash register and choose correct payment terms.