This excerpt taken from the CNET DEF 14A filed Apr 4, 2005.
Can I change my vote after I return my proxy card?
Yes. At any time before the vote on a proposal, you can change your vote either by giving our Secretary a written notice revoking your proxy card, by personally appearing at the annual meeting or by signing, dating and returning to us a new proxy card. We will honor the proxy card with the latest date. However, no revocation will be effective unless we receive notice of such revocation at or prior to the annual meeting.