How can we revoke a power of attorney and through what court do we do this process?

You need to do more than simply revoke a power of attorney. If your father is mentally competent, he can revoke the power of attorney and contact his financial institutions to inform them of the same. He can then start a lawsuit against your uncle for unjust enrichment to recoup the stolen money. If your father is not in a lucid mental state, and your uncle hasn't been appointed as your father's guardian by a court, you need to start a proceeding in Supreme Court for guardianship. If the court determines guardianship is appropriate, you will be able to act on your father's behalf, in which case you can do all of the above.