The PSR-E models are very menu-driven due to their small "dedicated" LCD screens rather than bit-mapped screens, so you must go into the Function menu for just about everything.
Perhaps the fastest way to get to the M.Octave function will be to hold down the DUAL button until the D.Voice function is displayed, then use the CATEGORY buttons to go backwards from there, which will take you through the various Main voice functions in reverse order-- M.Reso, M.Cutoff, M.Releas, M.Attack, etc.-- until you get to M.Octave. It might be quicker to go to one of the functions that are in front of the Main voice settings, but I can't remember which one that can be direct-accessed by holding down a button would be closest to the Main voice functions.