This excerpt taken from the CCO DEF 14A filed Apr 7, 2008.
Q: May I change my vote?
A:
If you are a stockholder of record, you may change your vote or revoke your proxy at any time before your shares are voted at the annual meeting by sending the Secretary of Clear
Channel Outdoor a proxy card dated later than your last submitted proxy card, notifying the Secretary of Clear Channel Outdoor in writing, or voting at the annual meeting.