JankBone like @Toxe says, change velocity to match the desired direction. How you do this is up to you - you could create a bullet scene variation and set its speed to -speed so it goes the other way (and use that in enemy), or perhaps have a start method that accepts a speed, called with the appropriate value, e.g. Vector2.LEFT (for the enemy) and then in start:
func start(Vector2 direction) -> void:
velocity = direction * SPEED
Or rotate the whole thing so velocity.x is the other way 😉
(also, you only need to do it once)
