This is very simple indeed once you know the trick.
Put the vacuum on a counter top.
Stand behind and to the right of the vacuum and with your left hand lower the vacuum as if you were using it. Hold it there.
On the back right hand side above the wheel there is a curved "airway inspection" pipe, take it out by squeezing the two clips fully together and pulling up.
Now you have a big square-ish hole in a part that moves. Mine is purple to match the vacuum. It is stuck in a down position jamming the stay-standing-up lock.
Put your thumb inside the square hole and lift up. You will find the whole thing moves in a backward curve, pivoting around the round hole to the left. You will meet resistance, folow through until it clicks.
Voila! The vacuum will now stand up.
Replace the airway inspection cover and smile.
Nick, June 2010