OK so I'm sure it's been asked before but if Boba Fett with navigator does a 2 turn can I change it to a 1 turn or can I only turn it to a 3 turn?

You can only change it to an option on your dial. For the firespray that is a 2 or 3 turn. If banking then you have the 1, 2 and 3 option.

You can use Imperial Boba Fett's pilot ability and the effect of Navigator in any order. So if you initially planned a bank maneuver, you have access to every bank maneuver on the dial.

However, if you plan a 2-turn, Boba Fett's pilot ability has no effect. You can only use Navigator in that case, so you can change your (e.g.) right-hand 2-turn into any other right turn. In the case of the Firespray, that means you can change it to a right 3-turn, but not into a 1-turn because the 1-turn isn't on your dial.

And yes, Navigator works exactly the same way with Stay On Target.