How can I show QR-Code of App Password AGAIN?

As a normal user in NC 24 you can set an app password (Settings->Security->Create new app password). Doing so there is offered to show a belonging to QR code. This QR code simplifies the setup of NC Clients (for mobiles) very much.

How can this QR code been shown AGAIN?

This would be helpful to setup the mobile client again.

You can’t. Create a new app password to get another QR Code.