Hi,
Please refer to
here page 15 (Install AT firmware).
To allow ESPresso Lite to enter flash mode,
1) Attach ESPresso Lite to UC00C(or any other USB-UART converter)
but left DTR unconnected (you can bend the DTR pin leg on ESPresso Lite slightly so that it wont connected to DTR pin of UC00C).
2) While holding GPIO0 button, press RESET button to reset it. ESPresso Lite should enter Flash mode by now. Then you can continue the steps mentioned from link above.