Quite simple. They simply snap in/out with the tip of a small
screwdriver. Pull the whole drive system down so you can work on it.
Pull the power plug, or turn off the breaker that serves the circuit.
Disconnect the wiring to the power head (note where the wires go!!),
disconnect the "l" shaped piece that connects to the door -- there is
a clevis pin and clip for this. Remove the retaining bolts for the
case. This allows you to lower the power head to the ground. You might
also remove the connection to the bracket over the door, if it is
awkward to work on. Lay it all out on the floor or your work bench,
and put in the clips. Total time, maybe 1/2 - 3/4 of an hour. You
will need a step ladder and socket wrench to remove the bolts to the
case, and some of the other parts. While you are at it, lube the
screw shaft, etc., which should be done yearly. They suggest a special
grease, but I just use normal automotive ball joint grease.