On page 41 of the PSR-E333 owner's manual it says you can decrement the function number by pressing the [ - ] button while holding down the [FUNCTION] button, so that gives you a way to move backwards through the function numbers if you've accidentally pressed the [FUNCTION] button too many times.
However, you can also jump to specific functions by either pressing or holding down certain buttons-- e.g., holding down the [SONG] button will take you directly to the "Song Volume" function, whereas holding down the [HARMONY] button will take you directly to the "Harmony Type" function. So if you want a way to quickly get to a specific function but there's no button that takes you there directly, press or hold down whichever button takes you to the function closest to the one you want, then use the [FUNCTION] button (or the [FUNCTION]-[ - ] combination) to go forwards (or backwards) to the desired function. For example, "Transpose" is function 3, whereas "Song Volume" is function 2, so you could hold down the [SONG] button to jump to the "Song Volume" function, then press the [FUNCTION] button to advance to the "Transpose" function.