Lemon juice! Not only does it help get rid of any current acne you may have but it also helps with acne scars. You have to use it daily and it will take a couple of weeks to really see results. I use it at night and put it on a cottonball. If you have sensitive skin you will probably want to dilute the lemon juice because it does sting a little.
Some other options are creams that you buy to diminish scars(mederma and such), honey, ice cubes, vinegar. There are a lot of different home remedies that can help you get rid of acne scars. But I have never tried any of these so I cannot vouch for them.
The idea behind non-deep scar removal is that you want to replace the skin at an equal rate as the surrounding skin.
Once our skin reaches it's adult period the process in which it replaces itself slows down drastically (because you're not growing anymore!) But if you're not willing to wait the years it will take to do it on it's on then thinning the skin through a light acid cream or causing it to exfoliate evenly will excel the speed at which it replaces itself but be done slowly enough so that it retains its natural look over the discoloration.
For deeper scars you cannot. The entire area is made a completely different way and there's no way to go back, yet!
You might be able to get a skin graft I think it's what they do to people who get 2nd and 1st degree burns but you can ask a doctor if it can be done for acne scars or scars in general than find out how much it would cost.
here is a link with a big list of treatments but I don't think they are the over night kind.