To show the qr code on the screen, you must do the same thing youd done when you first received the printer. That would be to log in on your printer.
Simply sign out of your account via the Display, and a QR code will present itself to link with your handy app and log you in.
Afterwards, you must log out and back in to your studio account to relink it.
Easy peasy. Its in the manual you threw out.