When you reach the platform, Caledonian Sleeper will have your updated booking information, it is important to present your upgrade confirmation email/barcode along with your original ticket confirmation that was sent by Caledonian Sleeper.
Once on the platform, there will be a host for each destination, go to the host that is showing your destination as per your original ticket. The host will then check your tickets and let you know which room to head to.
You will find your key card already in your room waiting for you
P.S. A little birdy told us that sometimes there is a bagpipe player on the platform, they don't take requests, unfortunately, but they should get you in the Scottish spirit!