The AT command is: BL. Note that the
bootloader runs at 115200 baud so it is best to first change your baud
rate to 115200 (ATBR7), do a write to flash (ATWR), and then enter
bootloader (ATBL). After you update the firmware, the default baud rate
of the application will be reset to 9600 so after exiting the bootloader
you will need to change the baud rate of your terminal emulator to
9600. Type “AT” with no command to see if it responds with “CMD OK” to
verify your current baud rate settings are correct.