How do I get a false proposed adult protective services report rescinded in California?

If APS has not done an investigation yet, nothing. You let them do the investigation and you will be contacted about their findings. The accusation may be unfounded meaning your innocent. You will need a lawyer to reverse any decision where you are found guilty.

With an attorney. Would say that's your best chance. But this isn't done lightly and wouldn't be removed without solid proof, either.

Once a report of suspected abuse.... and that is what it is until the report has been founded...is filed the investigation will proceed.
If it is actually false you have nothing to worry about.
If there is even a SLIGHT chance that the accusation will be founded you might want to talk to a lawyer.
If there is a very real probability that the accusation will be founded then you really need to talk to a lawyer.

You don't. The matter may or may not be investigated by APS and they will determine what's going on. This won't be the first, or last, false report they've seen, especially if dementia is involved.