WARNING: This page is about a different car, the 2008 Pontiac G5 and 2008 Chevrolet Cobalt. However, it is still accessible from the selected car via links, so may be relevant.
Fig 1: View Of Rearview Mirror Mounting Components
Courtesy of GENERAL MOTORS CORP.
Loosen the set screw in the base of the mirror.
Disconnect the electrical connector from the mirror, if equipped.