Depending on the type of Sonicare toothbrush you have, brush heads either snap on or screw on. For example, the DiamondClean, FlexCare, Sonicare for Kids and HealthyWhite models have snap-on brush heads, and the EasyClean, Essence and Xtreme models have screw-on bush heads.
Remove the old toothbrush head by snapping it off the brush handle. To do this, pull the brush upward until it snaps off. Snap the replacement head on by aligning it up with the metal rod and pushing down until you hear a loud click.
Unscrew the old toothbrush head by holding the bottom of the handle tightly while unscrewing the brush head, using the grooves as a grip. The entire top portion of the toothbrush should detach from the handle -- including the top piece with the brush head and magnet, as well as the plastic dome covering. Screw the replacement brush on by aligning the brush head with the hole and turning the plastic dome to the left until the entire brush head is secured.