You have to take the bottom and top shells apart to fix this - there is no other way. But it's really not very hard, IMHO.
Once you do so, it will be obvious how to remove the pin; in my experience, you can just push it out with the end of a small hex driver, and push it into the new arm with your finger at first. You may need to bend the surrounding plastic a little to do so.
You may need a pair of needle-nose pliers to GENTLY twist and work the black/blue/red wire connectors apart.
One last tip - don't forget to put the power button back in before re-attaching the top and bottom shells!