You should have received the firmware, instructions and stl files for the printer's printed parts on the SD card that came with the machine. You can also download here (no guarantees it's current, though). I would take Roxy's advice and take the values in Configuration.h from sd.zip (the file in the link) and transpose into the latest Marlin firmware. The firmware has changed a fair bit from one to the other, so it won't be a simple as overwriting one file with the other.

No you, can't export firmware from your printer as source code.