I could only find one advance arm but it'll still work to show how it's done.
Open the torso from the side. Easily peels off but still handle with care. You don't wanna rip the body.
Pull the torso muscle off. Comes off like a shell.
Unscrew the back of the body. You'll need a phillips screwdriver. There's three screws that need to be removed.
Should look like this with the back piece removed. Remove the arms.
Here's where the real modding begins. As you can see, the advanced arms have a bigger round peg. That peg needs to be either sanded or cut to the size of the muscle arms to fit back into the muscle body's inner socket.
Once you get them in place, just close it back up. They're a little tricky and might fall out. It's simple to get it shut again though.
Put the torso muscle back on and glue it shut. Don't add too much glue. Just in case you wanna undo the whole mod.
Here's the finished body with the arms.
Hope this helps.