I looked at the manual for an item called Yamaha TX81Z, I hope this is right for your module. Starting on page 67 there is a whole large section outlining how the parameters can be changed by sending System Exclusive messages over MIDI. Changeable parameters including voice edit (VCED), voice edit additional (ACED), performance edit (PCED) and many other types of parameter can be sent to the module in this way.
If you don’t already have this manual you can find it by searching on “Yamaha TX81 manual”. Manualslib is one website that will let you download it free, just pass the annoying “identify the crosswalk” test.