Some lever handles have a small set screw on the side of the handle where it attaches to the plumbing. Some require a small allen wrench to remove the screw and others require a very small phillips head screw driver.
Also, some are reverse threaded, so you have to unscrew it by turning in a clockwise direction.
Also, some are reverse threaded, so you have to unscrew it by turning in a clockwise direction.