Yamaha PSR540, voice change question

Yamaha Portatone PSR540, voice change question. I bought this keyboard new at least 10 yr ago for my daughter; don't think there is problem because she never used it at all. No detailed info on manual to understand why voice change is not working the way I can see. I'd really appreciate if someone can help.

Example is: Pressed ?voice change? button, chose voice (flute), pressed ?Exit?per manual; noticed ?voice change? button light came off; thought it?s set. Then, I chose another voice (piano) to play. So, when playing ?piano?, hitting ?voice change? button again to call up ?flute?previously set (voice change button light came on), hoping to change to ?flute?; but display only showed the new voice, i.e. ?piano? and not ?flute? originally set; in fact, the ?voice change?showed the ?piano?as if it were reset.

How can one change voice by pressing the one-touch ?voice change? button on such keyboard, or does it work that way? I can use Registration Memory or Multi-pad to play as alternatives; then I don't know what the one-touch voice change function is for. Can anyone help?

Thank you in advance.
Lou