I concur with Nathan's answer. It's typically not cost effective nor of much benefit to take the cage off of a rear derailleur at this (relatively low) level of Shimano hierarchy. New Acera derailleurs can be purchased for the mid $20USD range. If your time means anything to you and the fact of questionable benefit makes tearing into this a possible waste of time.
That said, (-: the piece you are trying to remove is a U shaped clip which retains the derailleur body to the bolt that is fixed to the inner cage plate. It does come out with some good persuasion. Inside this area is a spring, one end of which you'll see set into the inner cage plate behind the pulley wheel. Remove the outer cage plate by removing the pulley bolts. When the outer facing aspect of the inner cage is accessible, it might help to push the end of the P-spring into the derailleur body which will free the cage to turn. IIRC you still need to remove that retaining clip in order to remove the inner cage.
Inside you'll find just the P spring which is directional so keep track of the orientation. The spring surrounds a hollow axle of thin metal. The spring's ends set into little holes--one in the derailleur body and one or two in the cage. During reassembly you'll set the ends of the spring into their respective holes and wind the cage to generate the P-spring tension. It's a bit tricky to keep the ends of the spring engaged while keeping the cage out a bit from the derailleur body so that it can spin beyond the body's cage stopper. In the video, the stopper bolt can be removed, but at this level the derailleur body has a raised section molded into it or the stopper is a fixed pin that cannot be removed from the cage. So the technique is to wind the cage generating spring tension then when beyond the stopper bolt, push the cage and derailleur body together and reinsert the u clip.
