Looking in the manual, I don't see any Transpose buttons on the panel, so that means the only way to access the Transpose feature is through the Function menu. The thing that makes this troublesome is that (at least on the models I'm experienced with) whenever you press the Function button it takes you to the last function you'd accessed, which might be half the menu away from the function you're trying to get to, thereby forcing you to tediously page forward or backward through the menu to get to the desired function.
On some models you can shortcut to various points within the Function menu by pressing (or usually pressing and holding) a button that takes you directly to one of the functions. If you're lucky, there's a button that will go to the function you want, but most of the time you have to settle for shortcutting to a function that's close to the one you want in the menu. Unfortunately, the DGX models seem to be short on buttons that jump to specific points within the Function menu, since most of them seem to go to settings which are "off menu." However, it looks like the Chord Fingering button (which is of the press-and-hold type) will get you close to the Transpose function. So the quickest way to get there is to press and hold the Chord Fingering button, then go backward five (5) pages in the menu to get to the Transpose setting.
Note that if you're wanting to switch between different Transpose settings during a performance, it's really no different than wanting to switch between other settings-- voices, effects, etc. That means it might be desirable to set up some specific Transpose settings and save them to different registrations. Of course, since nearly all of the keyboard's settings get saved to and recalled from a registration, and there is no "freeze" option as on PSR-S and higher models, you'll need to set and save most of the other functions the way you want them to be when you recall a particular registration to recall a given Transpose setting.
But for transposing on the fly before starting a particular song, the simplest way will be to shortcut to the Chord Fingering and then step backward five times to the Transpose setting.