Unplug the mixer's power cable. It is dangerous to work on any high-powered audio equipment while connected to a power source of any kind. If the mixer has been running for a while, allow several minutes for it to cool down.
Remove each plastic knob from the mixing board. This is done by simply grasping each knob between your fingers, and lifting straight up until each knob disengages from the mixer. Store each set of knobs in separate plastic bags so you remember where each set of knobs goes. For instance, store the fader (volume) knobs in one plastic bag, and the EQ (equalization) knobs in another bag. This helps alleviate confusion when you put the mixer back together.
Loosen and remove the Phillips-head screws fastening the mixer's top cover in place. Grasp each side of the top cover, and lift straight up until the cover is completely removed. This will reveal the internal components of your Behringer mixer.