Hmmm?
You can create a global variable that reads max speed.
MaxSpeed = player max speed
Then add whatever speed you want to increase that to.
On bullet collision with player: Set player max speed= Maxspeed+500
Something like that but not sure if it is the effect you want. It would depend on if you want that a sustained change or just temporary effect.
If temporary add in a wait for so many seconds and then reset the player speed to Maxspeed.