Upgrading My MacBook Pro (Late 2013) Components
There's not much you can do, other than change the SSD. Apple solders essentially everything down onto the motherboard (CPU, RAM, chips...). Unless you have insanely good microsoldering skills, you can't swap the CPU or RAM. 

The only think you could upgrade would be the SSD drive, which should be a M.2 PCIe drive, like this. Even then, you'll need a Pentalobe screwdriver.

If you're interested in how irreparable your Macbook is, here is the teardown from iFixit.