This excerpt taken from the PRVT DEF 14A filed Nov 3, 2009.
How can I change my vote?
If your shares are registered directly in your name, you may revoke your proxy and change your vote at any time before the Annual Meeting. To do this, you must do one of the following:
If your shares are held in street name, you may submit new voting instructions by contacting your broker, bank or nominee. You may also vote in person at the Annual Meeting if you deliver a legal proxy as described in the answer to the How do I vote? question above.