What I used was a pair of disinfected dog/cat nail trimmers. I also "hooded" the chick to minimize stress (covered her eyes and flipped her over) and also some blood clotting silver nitrate (for when the beak bleeds after trimming).
For my older pullet 8weeks she has a nearly totally opposing beak 90deg, I trimmed both her top and a little off of the bottom (just a tiny bit to help the top and bottom match up better). She is very active and 2 weeks later she eats her chick starter mash right up with little to no difficulty, and forages and pecks at scratch with the other chickies. She is not picked on and growth wise is just a tad behind the regular beaked chickens.
Just don't forget the quick stop/silver nitrate and to disinfect the trimmers.
If the top beak is just a little crossed, try to just trim a little and leave the bottom alone. I would not trim the bottom unless it was absolutely necessary for eating. The bottom beak is how they scoop water and food so it is the important part of the two beaks.
I also have a little 3 day old that I noticed a VERY slight mismatch and I went ahead a trimmed the top beak just a tad and her beak is meeting up well. I will keep trimming if needed as she grows, and I keep an eye on both of them to make sure they drink and eat mash well.
Good luck and keep in touch....
-nj