Genghis/kenpimental
Looks to me that the transponder control is the small box just above the blank panel near the bottom on the right hand side. In which case, it's a case of dial each number in
That's certainly where the controller sits but I'd point out that the exact choice of panel is a customer option - I certainly don't recognise the one in the flight deck overview image, and I don't recognise the one that kenpimental has provided a link to. All our 777s (200s and 300s) have a different panel design, they have a small keyboard for number entry, so you punch the code in rather than dial and FWIW on ours there's no need to select STBY before changing code.