how do i fly faster?

Everytime, i fly any of the boeing planes around 35,000-39,000 ft. i get a message that says 'overspeed' when i'm flying at 300 knots, is there any way i can fly faster then that? because this one time, i was flying the F-15 jet following a boeing airplane and they were going faster then 300 knots at the same alt. i was flying in 
