You cannot upgrade these machines in a practical fashion. The SSD card can be swapped out but the RAM is not upgradeable. As for that SSD card, getting to it without destroying the machine is something that no repair shop will attempt.
About an hour ago, I ordered my 3rd Surface Pro 3 and you can still find a few, new machines out there and it has those specifications you desire.
Here is the link where I ordered it from: https://www.walmart.com/ip/Microsoft-PS2-00017-Surface-Pro-3-Tablet/46258891
Other than that, I would say to look into getting the newer Surface Pro with the RAM and storage that you want because an aftermarket machine like the one I ordered will have no warranty and no support (even if it is new, sealed box).
Your current machine barely has enough resources for upgrades but should still be usable for basic things.