Can't remove the head rest to install our car seat cover

Head restraints are the double-post type that are adjustable up or down. Only the solid-type head restraints have serviceable trim covers.
Use J 42372 in order to remove the head restraints. See Special Tools. The rear seat head restraint has only 1 locking post.
Fig. 247: View Of Head Restraint Post Components
Courtesy of GENERAL MOTORS CORP.
Raise the head restraint to a full up position.
Push the head restraint down just far enough in order to center the locking clip (4) in the guide between the detents on the posts (3).
Insert J 42372 down the detent side of the post (3). See Special Tools.
Hold the tool down, and at the same time, lift the head restraint upward in order to disengage the detent (3) from the guide (4).
Lift the restraint out. Tools shown in pic you might be able to make one from sheet metal

You can take your car to any competent mechanic an d he can fix the lock to work