You can change the gear by disable the the collision groups from the shifter
so:
if you disable E you go into 1st gear, so E is your Neutral gear.
F is your first gear so if you disable F you go into 2nd gear.
G is your second gear so if you disable it you go into the 3rd gear.
H is your 4th gear so if you disable it you go into the 5th gear.
you cant disable the 5th gear, but you can go back into neutral by enable E.
if you will shift back down to 1th gear, i would recomend you to enable from H to E or F (what ever you want)
so i recomend you too that you dont skip any gear when you shift back down.
Controlls:
/\ = Throttle
Q = Toggle Brake
S = Starter
I = Igniton
I'm pretty sure you will figure out how it works.
Now have Fun!