I bought this bike as a cheap fixer upper for my daughter. The bolt was missing from the top so i don't know what to do. I was thinking of putting a long screw driver down there and hitting it a few times but i decided to fill it with lubrication spray from and then post here too see if anyone might have a better idea.
The stem will have either a wedge or conical expander at the bottom. You should be able to shine a flashlight in to see what type. If a conical type you will just see a circle of metal with a threaded hole in the middle. Just use an old screwdriver or similar and tap firmly with a hammer, but make sure you avoid the threaded area. If it's the more common wedge type I would say to place the screwdriver at the outside of the lowest part of the wedge and tap there. You may be able to find a replacement bolt at a local shop or co-op - needs to be the correct length and threading. If not you will have to replace the stem, which will have to have the correct diameter as well as the proper clamp size for the handlebar.
lean the bike over at an angle and drip some 3 in 1 oil down the walls, let soak for a little bit, then tap right on the top of the remains of the bolt. Once it is loose, you should be able to pull the stem off - but you may have to tap it down a bit to free up the works. After you pull the stem off, turn it upside down and the wedge should fall out. May need a bit of encouragement from the bottom.