The old 33® reel: Grip the reel tightly around the main body. Rotate counterclockwise. The back cover will become free. To reattach, align the back cover notch in the slotted area and turn approximately 45 degrees clockwise until cover is tight.
The new 33® reel: To remove the back cover on a new 33®, 33® Max, or 33® Micro, insert a small screwdeiver in the gap between the back cover and the top of the thumb bar and gently pry up. The back cover will pop off. To reattach, align the top of the front cover in the body groove and snap the bottom of the cover onto the tab on the body by applying a steady inward force.