Arthur, firmware is not something to play with. Improper install can result in permanent damage to printer.
Dont see how firmware was corrupted. Especially if printer was working ok prior cleaning. Be very careful here.
Firmware available here:
https://epson.com/Support/Printers/All-In-Ones/XP-Series/Epson-XP-850/s/SPT_C11CC41201
Be sure your operating system is correct in small window at top of this download page
Just click the .exe file with printer power turned on and printer connected to computer.
Do not interrupt this install process.
Might be better to
first try re-install of printer using Epson install CD. You must
first un-install printer.
Should you be using Windows operating system, follow this:
Control Panel -> Devices & Printers ->
Right Click XP-850 Icon -> Select Remove Device
Disconnect computer from Internet. Disconnect Printer USB connection until instructed to connect.
Now proceed with printer re-install. Follow your Epson fold-out instructions for install process.