WARNING: This page is about a different car, the 2005 Buick LaCrosse and 2005 Buick Allure. However, it is still accessible from the selected car via links, so may be relevant.
Fig 1: Locating Water Outlet Bolts
Courtesy of GENERAL MOTORS CORP.
Remove the water outlet bolts.
Remove the water outlet.
Fig 2: Identifying Water Outlet Gasket & O-Ring
Courtesy of GENERAL MOTORS CORP.
Remove and discard the water outlet gasket and O-ring.