User replaceable? No.

Brave hacker replaceable? Hell YES!

http://liliputing.com/2015/03/upgrade-a-surface-pro-3-with-a...

http://surfacepro3ssdupgrade.blogspot.mx/2015/02/surface-pro...

Short answer: No.

Long answer: The Surface 3 Pro had an SSD that could in theory be upgraded, but it required some serious disassembling to pop the display off, and even the ifixit guys couldn't pull it off without breaking at least one screen.

https://www.ifixit.com/Teardown/Microsoft+Surface+Pro+3+Tear...

Assuming it's similar to the rest of the Surface line (which seems a safe bet), the answer would be no but you can expand the storage with an SD card.