You can find it on the Asus site ("Asus S56CB Ultrabook User Manual"), or looking for "User Manual" on several documentation sites. The disk replacement instructions are at page 64.
There are two screws on the bottom front panel, and once removed, the hard disk is exposed. It is locked with four other screws and can be lifted by gently pulling on its front tab.
The procedure for mSATA should be here (with pictures), with some caveats:
There is only one screw holding the 24GB mSATA SSD to the motherboard. Once removed, set the screw aside and the mSATA SSD will pop up, just like a stick of ram.
Test fitting the 128GB Crucial mSATA SSD, a problem arose. The standoff used to secure the 24GB integrated SSD is getting in the way! The standoff will need to be removed.
The bad news:
** WARNING ** Removing the standoff that secures the OEM 24GB mSATA SSD to the PCB will instantly void the warranty. It won't damage the computer, as long as you are careful, but you can consider your warranty gone once you do this. To remove the standoff, I used a pair of bent nose pliers to firmly grasp the aluminum standoff, taking GREAT CARE not to scrape, contact, or otherwise disturb any part of the PCB or motherboard circuitry; once the standoff is grasped, twist clockwise or counterclockwise (again, CAREFULLY), and it should come off, relatively easily. You don't need herculean strength to twist it off, just firmness, and finesse.
The good news:
The guy apparently has now a Crucial 128GB instead of the original mSATA accelerator.