I see this was posted almost 2 weeks ago, but maybe it still helps.
Even if the update fails, and powering down might lead the firmware to end up in a corrupted state, EPSON makes things easy for users.
They have a Firmware Recovery Tool available on their website.
That works really well, and helped me save many client devices already when they had the same issue.
For me in EU it would be this link [link|https://www.epson.eu/en_EU/support/sc/epson-ecotank-et-2850/s/s1953?suggestionClicked=true|Epson EcoTank ET-2850 Support|new_window=true], depending on your location you might need a different link.
On that page you can click on firmware, and download the tool to recover your device.