How do I change the days in procedure (D.I.P./DIP) of an invoice?

Look up the invoice via the search bar OR
Go to Clients and navigate to the concerning debtor
At the bottom of your debtor page, you will find all debtor invoices. Go to the invoice for which you want to adjust the D.I.P.
On the right you will see the D.I.P. of each invoice.

Use the pencil to change the number and then press enter on your keyboard.