Rotate the ball cover until you feel resistance. Depending on the brand of mouse, this could be clockwise or counterclockwise.
Pick up the mouse and flip it over into your other hand. The cover and mouse ball should fall out of the mouse. If not, give it a little shake until it comes loose.
Note: Optical mice, which use a small laser to track movement, do not have mouse balls or rollers and do not need regular cleaning.
