WARNING: This page is about a different car, the 2006 Acura MDX, 2005 Acura MDX, 2004 Acura MDX, and 2003 Acura MDX. 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 Size: AA
Confirm that each operation works normally after the batteries are replaced.