Adding an ISP port - dkidd928/cartreader GitHub Wiki
You can add a proper ISP connector to your Cart Reader by ordering this 10pin-to-6pin adapter and soldering it to the backside of the Arduino: https://www.ebay.com/itm/163202039261
- Cover the underside of the clock generator with some tape to prevent short circuits.

- Pull off the plastic of the 6pin connector then desolder all the little legs.

- This is the pinout of the ISP connector on the backside of the Cart Reader.

- Connect the ISP Adapter to the Arduino's ISP.

- You can use hot glue to hold the ISP connector in place.

- 
Download the USPASP drivers from here: https://zadig.akeo.ie/ 
- 
Change the Arduino IDE's programmer to USBASP 

- To program you might want to remove the battery first.