I'm trying to reflash the Expansion board 3.1 firmware and hence going through the docs at website/github.

There it states (highlights mine):

Windows

Download and install DFU-util v0.9

For Windows, we will need to install separate drivers for the board to recognized as a Pycom board in DFU mode.

1. Disconnect the USB cable to your expansion board
2. Hold down the DFU mode button on the shield

• Zadig – Installer tool for the Pytrack/Pysense board DFU Firmware

To install the drivers, the board must be in DFU-mode:

1. Disconnect the USB cable
2. Remove the development module from the expansionboard.
3. Hold down the button on the shield
4. Connect the USB cable
5. Keep the button pressed for at least one second

Which button is mean by "the DFU button" or "the button"? I have two on my board.
This should be more clear.

Thank you in advance.