Because Denon’s servers no longer officially support legacy products like the AVR-591, you cannot simply press “Update” over Ethernet today. The automatic update servers were decommissioned in 2018. However, the file still exists in community archives and can be applied via USB.
Disclaimer: Denon no longer supports the AVR-591. Any firmware update performed manually is at the user’s own risk. Always back up your original settings and ensure stable power during the update process. denon avr591 firmware update patched