Insert the USB into the front panel of the AVR-E300.
Download the firmware file (often an .img or .bin file) from the Denon Official Support Site to the root directory of your USB drive. Denon Avr E300 Firmware Update
Ensure your receiver is connected to your router via an Ethernet cable (highly recommended for stability) or a strong Wi-Fi signal. Insert the USB into the front panel of the AVR-E300