I have Navision 2018 and have updated the due date in a posted vendor invoice but can't seem to figure out how I did that as I is not letting me do it again. I tried the F2 trick and am a Super User.
You cannot change a posted invoice, including it's due date. So if that's what you're trying, then the "F2-trick" will not work.
But what you (normally) can change is the due date on the vendor ledger entry, the date that is used to manage payments.