Your options are quite limited! If you have a Core i5 1.4 GHz you could replace the logic board to get a more powerful CPU Core i7 1.7 GHz. As the RAM is also soldered on the logic board you could also get a replacement board with 8GB as well.
The SSD drive is upgradeable on its own! You can find Apple custom SSD’s as well as third party from OWC or Transcend which both make the fully compatible SSD’s. Using anything else just won’t last as they don’t fully integrate to Apples interface.
The graphics and display is not upgradable. The WiFi & Bluetooth are the same for both boards. So its is what it is. I really doubt you need anything faster given the generation of the CPU’s.