Is there a way to edit how fast an airplane can go? I tried changing reference speeds in the .air folder but it didn't seem to do much. Any help would be great.

I believe the speed of any given aircraft model is determined by the designer/builder, and from what I've seen, is probably a real world figure.  However, Microsoft has provided for easy mods, and somewhere on the MS site there, maybe even this one, is a write-up on many aspects of what does what as regards the .cfg file. For now:

In the aircraft.cfg file:

[flight_tuning]
parasite_drag_scalar = 1.0:  A lower value increases speed, a higher value decreases speed.

[jet_engine]
thrust_scalar = 1.0: A higher value increases speed, lower value decreases speed.

1.0 is the standard entry on these lines.  Watch your decimal places.  For ex., 0.8 is lower than 1.0.  You do not have to modify both.  If an aircraft does not have these lines, then it is defaulting to 1.0 values.  You can add these lines if necessary.

I have not noticed any adverse effects on any aircraft with these modifications. It is, of course, more difficult to fly at higher speeds, but you can always change the mods back.  Don't forget the "=" sign.