As far as I'm aware, Yamaha themselves never released a style editor to the public.
I haven't worked there for 20 years, but I don't think that has changed in the intervening period.
I used to, in the 90s, actually make these things for a living ;-)
I was part of Yamaha's writing & mixing/editing team who made the internal styles & a lot of the first release-cycle of the then-known 'disk styles' you could buy, initially for the high end products such as the PSR-6700, right up to the last before I left, the PSR-9000 & Tyros. Lower models would be handled by another team, who trimmed down, re-voiced & re-edited from the data we made for the top models.
So, my information is 20 years out of date, but these styles were always actually made in a static C Maj7 chord. The note function & movement was then re-calculated live during playback. Using C Maj7 as the base data gave each note a 'function' from which the ABC could be calculated. Root, 3, 5, & "flavour". There were some rather sophisticated algorithms & different rule-sets which dictated which note went to where on transposition, & these rules could be set per track/instrument by the programmer. Whether it was simple transposition of the entire chord - the simplest was to transpose in parallel, up until a certain key, then wrap over to transpose down [dumb but sometimes useful]. Much smarter algorithms could dictate quite clever re-voicing to make a player's voicings always appear to move to the nearest 'good' note for the chord, with correct voice-leading etc. Even more complex algorithms for other scale & non-scale notes were capable of keeping the whole thing sounding reasonably musical.
Unfortunately, the exact details of how this was done [because no other company had managed to achieve it] I have to keep to myself even to this day.
Anyway, the background aside, I still don't think there is a dedicated publicly-available Yamaha resource to handle these files at consumer-level. All 3rd party software will have had to try to reverse-engineer how the algorithms are applied.
The best resource I know of for this 3rd party software is curated by a chap called Jørgen Sørensen on his site - The Unofficial YAMAHA Keyboard Resource Site On two pages he has links to all the known editors for Yamaha keyboards - http://www.jososoft.dk/yamaha/software.htm [& see the header bar for 'Software 2'].
I haven't tested any of this myself, but I feel sure this is going to be your best resource.