The MacBook Pro Retina uses a solid state drive (SSD). Are you sure you have this model of MacBook Pro?
Here is the iFixIt breakdown of the MacBook Pro Retina system showing the removal of the SSD module Step 10. This is a custom designed unit and presently Apple is the only supplier. As such your choice is limited to one of two sizes 256 or 512GB at the time of your systems purchase.
I'm not sure I understand your question on upgrading the OS as the MacBook Retina has the newest (current) version of Mac OS-X on it.
If you require additional storage you'll need to get either a USB3 or Thunderbolt connectable external HD
Well, yes, and no. The new drives are proprietary, if you install a non OEM you'll have to use a fan control app to keep the fans from running away. OR do some hardware rewiring. IMnsHO it's better to just order the larger(est) HD with these new Macs.
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