How can I get guardianship for my Dad when his sister has it now?

A Guardian is appointed by a Surrogate's Court Judge and turns in annual financial reports. You would file a written complaint with the Surrogate's court to remove a Guardian. A POA is different usually appointed by the patient himself. If your father is mentally incompetent and you have proof that his funds are misdirected, call the police.

I assume you mean your aunt has a Durable Power of Attorney.

What's your father's situation? Is he living with your aunt, in a facility? What are his medical conditions and is he suffering from any dementia?

I assume you have written proof of the "neglect" of his funds and misappropriation for aunt's own use?

Others here are more experienced with guardianship, but I believe you'll have to file a petition with your county Probate Court in order to get guardianship. However, given that there are apparently disagreements and now the accusations within the family, the Court may very well appoint an independent guardian, to be paid from your father's funds.