My father's current Power of Attorney is not being financially responsible. How do I change his POA?

Only your father can change his designation of POA. It is a "donation", according to the law, and it ceases with the death of the donor.....lots of paper work involved here. See a lawyer.

If medically competent get a new power of attorney appointed. We had to have my uncle do ours as a neutral party to keep my sister and I out of the loop. If you have a relative that you trust to handle these things, by all means do it. My sister hid all my mother's money and after the POA change we had the department of aging investigate her and she had to give the money back that she used. She had 5600 unaccounted for as well but if you suspect elderly financial explotation, by all means call you local dept of aging. First and foremost, get that POA changed immediately.

There are two of us as POA for my mother-in-law, but it's only ME that runs the checkbook and pays the bills. If you're afraid to subtract your brother, then maybe that's how you can work it out. Add you and your sister then tell brother that you or your sis want to start paying the bills. I also have all my mil's bills coming to my house too. The only mail she gets in the asst living place is junk mail (very little junk though) and personal mail.

You need your father's permission, if he is of sound mind. If he has been medically declared incompetent, I do not believe that it can be changed.
The best thing to do is speak with an elder attorney and explain the situation.
good luck