Power off the Denon AVR-X520BT completely using the power button.
If things don't go according to plan, use these "field-tested" fixes: Firmware AVR-X520BT Denon Avr-x520bt Firmware Update
If the update remains stuck on the same percentage for more than 30 minutes, a file corruption issue may have occurred. Power off the Denon AVR-X520BT completely using the
Copy the extracted files of the USB drive. Do not leave them inside a subfolder, or the receiver will not find them. Safely eject the USB drive from your computer. Phase 2: Running the Update on the Receiver Denon Avr-x520bt Firmware Update