What did the government supposedly lie about regarding United 93?
That's a complicated answer. From what little I could stomach listening to a conspiracy nut on NPR, they believe the CIA hijacked the planes, not terrorists, and switched them midair with UAV's that LOOKED like the planes and then crashed the UAV's into the World Trade Center, the Pentagon and the field in PA.
They then flew the real airplanes out over the Gulf and exploded them with all the people aboard.
This basically denies the heroic first stand the passengers of 93 took against the terrorists to save the White House, or the Capitol.