Open up /etc/sane.d/snapscan.conf in your favorite editor:
sudo pico /etc/sane.d/snapscan.conf
At the top of the file, you should see something along these lines:
# Change to the fully qualified filename of your firmware file, if
# firmware upload is needed by the scanner
firmware /usr/share/sane/snapscan/your-firmwarefile.bin
Make the filename/path of the line that reads firmware /usr/share/sane/snapscan/your-firmwarefile.bin point to your firmware file. Make sure the line doesn't start with a hashmark (#); if it does, remove that character. Following our example, this line should read:
firmware /usr/share/sane/snapscan/u176v046.bin
Save changes and close the file (if you're using pico editor, press CTRL+X, it will ask you whether to save the file: press "Y" and hit ENTER)