WARNING: This page is about a different car, the 2006 Honda Odyssey and 2005 Honda Odyssey. However, it is still accessible from the selected car via links, so may be relevant.
Remove the cover (A) from the remote control.
Fig 1: Removing Cover From Remote Control
Courtesy of AMERICAN HONDA MOTOR CO., INC.
Remove the batteries (B).
Replacement Battery: BR3032
Confirm that each operation works normally after the batteries are replaced.