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[Link: usa.denon.com]From Page 29 of the manual, paraphrased:
Press the System Setup Button -- the Setup Lock Screen should display -- select OFF and press ENTER.
It makes LOTS of sense to lock a setup. People who are idly interested in the menu system, but don't know enough to turn off the lock, should probably not be fooling around with any settings.
Installers can't possibly know all the methods of unlocking different brands, so this question is totally reasonable. But when they get in there, they are unlikely to mess it up as the casual user would.