How do I get power of attorney away from my stepmother?

You don't. If your stepmother has POA, it is because your father when he was still competent gave that authority to her. Only he can change it, and if his dementia has advanced so that he is no longer competent (this is competency in the legal sense that we're talking about, meaning of sound mind, able to understand the implications and factors involved in decisions) then he cannot change or create a new POA.

You say you feel like your stepmother is not caring for him properly. Could you describe what is going on? If there are serious problems, posing a real risk to your father's best interests or welfare, then there are other things you can do - but they are expensive and, yes, will bring you into direct and potentially hostile confrontation. So are you sure you're picking the right battles? Focusing on maintaining good communications and a constructive relationship with both of them is likely to be a much better way forward unless you have real, well-grounded concerns about his welfare.