Can I upgrade my MacBook Pro Retina?
Unfortunately I cannot comment on the other answer.  Ian is correct in that the drive inside the new rMBP's are solid-state drives, however, you can upgrade the drive to a larger drive yourself, but in doing so you may void the warranty so make sure that you verify that first, otherwise, you can take it into an Apple Store and they likely will upgrade it for you.  Either way, this will only give you more space and chances are will not increase your read/write speeds.

Here's the link to the instructions on replacing the SSD.

https://www.ifixit.com/Guide/MacBook+Pro+13-Inch+Retina+Display+Mid+2014+SSD+Replacement/27849

Your only obstacle might be finding one.  Currently, the SSD's that Apple uses are made by Samsung and use a PCIe connection.  They are very difficult to find as they are not yet on the mainstream market.