Here's how you do it.
Push in on the magazine catch stop with a small flat-blade screwdriver or similar tool, lift the assembly out of the frame. There are four pieces; the mag catch, mag catch spring, mag catch stop and the catch stop spring.
Take the grip panels off. Slide the support plate back toward the rear of the gun and re-install it on the opposite side.
Install the mag catch w/spring. With the pistol lying on your working surface, carefully install the catch stop and catch stop spring into the mag catch. The little "step" or leg on the catch stop MUST be at the 6 o'clock position (closest to the frame). Push the catch stop in while pushing the mag catch in as far as it will go. Carefully release the mag catch.