For instructions to remove and replace the fixed magazine (either for cleaning or to replace a worn or defective unit) on a CGAR15FM lower receiver, watch this video. (We no longer recommend retracting the follower as shown in the video. It is easier to simply remove the bottom plate on the magazine and release the spring.)
Note: Since this video was made, we've stopped recommending the use of special tools to depress the magazine follower. It's actually quite easy to remove the magazine floor plate and take out the follower and spring during magazine installation or removal from the lower receiver. The other special tools and the technique shown for installing the retention nut and screw are still the recommended procedures.
For a great video on how to replace the takedown pin without removing your stock or the castle nut securing the buffer tube, watch this YouTube pin replacement video.