A: Not from Upsales. Change "Our reference" (Vår referens) on the invoice in Fortnox. The user on the order in Upsales will then update.
This works when Modul att synka in the Fortnox app settings is set to Faktura or Order & Faktura.
The name in "Our reference" must match the name of a user in Upsales. If no user has that name, the order gets the user selected as Uppsamlingsanvändare in the Fortnox app settings.