Ensure your receiver is connected to your router via an Ethernet cable (highly recommended for stability) or a strong Wi-Fi signal.
If an update is available, the screen will show the estimated time. Select Start to begin. Denon Avr E300 Firmware Update
Insert the USB into the front panel of the AVR-E300. Ensure your receiver is connected to your router
While pressing the STATUS and TUNER PRESET CH – buttons on the main unit simultaneously, power on the unit. Denon Avr E300 Firmware Update
If your network is unstable, you can use a USB flash drive (FAT16 or FAT32 format).
Download the firmware file (often an .img or .bin file) from the Denon Official Support Site to the root directory of your USB drive.