By default the initial firmware has already been downloaded in the flash. If users need to re-download the firmware,
they should follow the steps below:
1. Set the module to UART Download mode.
2. Pull IO0 and IO2 low.
3. Power on the module and check through the serial terminal if the UART Download mode is enabled.
4. Download the firmware to flash using the ESP Flash Download Tool.
5. After downloading, keep IO15 low and pull IO0 high to enable the SPI Boot mode.
6. Power on the module again. The chip will read and execute the firmware during initialization.