There are a few ways.
One is to give the the object the bullet behavior and add an event like this:
--- sprite: rotate 10*dt degrees toward -90
Where 10 is the turning speed in degrees per second and -90 is the end angle.
Another way that is useful from a trigger is to add a instance variable "turningSpeed" to the sprite to tell how fast it turns.
--- sprite: rotate clockwise self.turningSpeed*dt
start of layout
--- sprite: set turning speed to -90
--- wait 1.0 seconds
--- sprite: set turning speed to 0
--- sprite: set angle to -90