I assume that (as on my PSR-E and YPT keyboards) pressing the FUNCTION button takes you back to whichever function you last accessed in the menu-- unless you've just turned on the keyboard, in which case it takes you to the first function in the menu. Since the YPT-320 has 34 functions in its menu, you might have to press the FUNCTION button anywhere from one to 34 times to get to a particular function.
However, there's a way to jump directly to specific functions in the menu, although you can't jump to any function you want. Some buttons take you to specific functions in the menu, so you can press-- or, if applicable, press and hold-- whichever button will take you to the function that's nearest to the one you want, then proceed from there until you reach the function you want.
For instance, the Main Voice Volume or M.Volume function is two functions after the Touch Sensitivity or TouchSns function. Rather than pressing the FUNCTION button over and over again until you get to M.Volume, you can press and hold the TOUCH button to go directly to the TouchSns function, then press the FUNCTION button twice to advance to the M.Volume function.
By the way, I don't see anything in the PSR-E323/YPT-320 Owner's Manual about going backward in the menu, but it's possible on the PSR-E333/YPT-330: "You can also decrease the Function number by one when you press the [-] button briefly while holding down the [FUNCTION] button." You might want to try that and see if it works.