This excerpt taken from the GOOG DEF 14A filed Mar 25, 2008.
A: You may change your vote at any time prior to the taking of the vote at the Annual Meeting. If you are the stockholder of record, you may change your vote by (1) granting a new proxy bearing a later date (which automatically revokes the earlier proxy) using any of the methods described above (and until the applicable deadline for each method), (2) providing a written notice of revocation to Googles Corporate Secretary at Google Inc., 1600 Amphitheatre Parkway, Mountain View, CA 94043 prior to your shares being voted, or (3) attending the Annual Meeting and voting in person. Attendance at the meeting will not cause your previously granted proxy to be revoked unless you specifically so request.
For shares you hold beneficially in street name, you may change your vote by submitting new voting instructions to your broker, bank, trustee or nominee following the instructions they provided, or, if you have obtained a legal proxy from your broker, bank, trustee or nominee giving you the right to vote your shares, by attending the Annual Meeting and voting in person.