Buy one USB to TTL device, Internet is plenty of them. Here is the link for the one I use. It's more expensive but it's protected against dust and comes with the cables. It will only be used to open a backdoor for later connecting through network so the cheapest one it's ok I guess.
Here comes the dangerous part
 | Identify the VCC pin in your USB to TTL device and do not connect anything there during the whole process |
Locate three pins in the SyncModule with the name J3 in it
Identify GND, RX and TX in your USB to TTL device and:
- Connect your RX pin to the Sync Module TX pin
- Connect your TX pin to the Sync Module RX pin
- Connect your GND pin to the Sync Module GND pin
You can see my connections here
You are done! Let's test the connection