On the recent airing of the Flight 93 documentary....a pilot (I believe) specifically said the terrorists did not know how to work the GPS on the plane, and didn't know how to turn off the alarm when the auto pilot is turned off....
That was an interesting documentary.
It would take many hours of training to learn how to operate the FMS in any airliner.
Turning off the alarm is simple once one knows. It's a simple push of a button if one understands the system.
I thought the documentary said he did program the FMS to for direct to his target-- but he did have problems with aircraft control-- kept disconnecting the autopilot and then reconnecting after he had trouble.