Please ensure the power strip is not plugged into mains power while soldering or flashing!!!
To connect a serial connection you do need to open up the power strip and solder on some cables to allow connecting your flashing tool / software of choice.
The TYWE3S chip is located on the hidden side of the board (see image below), so you’ll have to unscrew the PCB as well and turn it around.
As usual you’ll have to connect to VCC/3V3, GND, RX, and TX to flash.
Once connected you will also have to push the button located on the PCB (see picture below) to be able to re-program / flash with Tasmota.
Be careful not to pinch any cables when closing up power strip again after flashing.
Note: Circuit actually contains a fifth relay on the common neutral wire that needs to be turned on whenever one of the 4 relays/outlets is turned on.