This guide explains how to replace the release button if it is broken or not working.
Press the release button while pulling up on the head of the vacuum to remove the head.
Using a Phillips head screwdriver, unscrew the four screws holding the panel to the shaft of the vacuum.
Pry the panel up using a spudger and pull up firmly just enough to see the clips of wire on the sides.
Remove the wire clips on both sides of the shaft.
Pull the top piece out of the shaft.
Pinch the two pairs of plastic catches (circled in the picture) to release the button.