Increase jump speed colliding on a box ??

You should checkout the Unity documentation for some of the methods you can use. But for your question, your code should look like this:

 class box : monodevelop
 {
 public int extraSpeed;
 int playerJumpSpeed;
 bool jumpedOnThis;
      OnTriggerEnter()
      {
      if (jumpedOnthis == false)
      {
      playerJumpSpeed += extraSpeed;
      jumpedOnThis = true;
      }
      }
 
 
 }