Don't screw 'em up

When I asked an American friend to pass me a screw driver to repair my motorbike, he said with a laugh:

 Don't screw it up!

I could not understand why. What that screwdriver was for without using it to screw either up or down.

That was until the motorbike I was trying to repair became a real mess and needed a professional hand.

Why couldn't he just say "don't mess it up" instead.