Generally you can update firmware by using over the air firmware update, so there is no need to connect anything to the board. However, if you have to use your usb-serial connector, then as I said above, you can solder pins onto the serial pins. The connector we use is a 2×3 goldfinger connector. But you can certainly just solder pins onto the connector pads. On the front side, the pads are TX (i.e. ESP8266’s TX), +5V, GPIO0, on the back side are RESET, GND, and RX.