Xerox DocuMate 765 User Manual page 107

Scanner Options, Maintenance and Troubleshooting
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4. Place your left index finger on the outside of the bar top background
and push inward against the spring. Keeping the bar pushed in, use
your right hand and rotate the bar forward to flip the background plate
to the opposite color.
5. Remove your left hand to release the bar and lock the background
plate in position. Verify that the bar is locked in place before
continuing.
DocuMate 765 Scanner User's Guide