First verify that none of the spokes is broken. Next, if you don't have a spoke wrench, remove the tire and the "rim strip" covering the heads of the spoke nipples. Generally the nipples will have slot for a standard (though thin but with a wide blade) screwdriver.
Find a suitable screwdriver (try to find one that fits well, wide enough to fully engage both sides of the nipple, with the blade fully into the slot) and see if you can turn any of the nipples at all. (Sometimes the nipples are corroded onto the spokes so badly they won't turn.)
Next, you have to THINK about it. The spokes alternate sides, one going to the left side of the hub, the next going to the right. If the rim is bending too far to the LEFT in one spot, you want to tighten the spokes nearest to this bend that are on the RIGHT side, while loosening slightly those on the LEFT. This is, as they said in "M*A*S*H", "meatball surgery" -- you're not going to achieve a straight rim, but you want to get it straight enough that it can turn without rubbing, then leave the rest to a bike shop.
If the lip of the rim is bent (usually outward) from hitting a bump, you can straighten it to a degree using a "monkey wrench" or stout pliers -- tighten the jaws directly over the bent spot and bend inward or outward as needed. Use a bit of cloth over the outer rim surface to minimize scratches on it that will cause braking problems.
Be sure to replace the rim strip (or cover the nipple heads with some sort of tape), since un-covered nipple heads can put a hole in a tube in remarkably little time.
In a pinch, yes, you can run a smaller wheel on a bike. You will, of course, not have brakes on that wheel (and you may in fact have to loosen the brake cable all the way or some such to keep the brake pads from rubbing on the tire). Be careful braking, and don't do any long downhill runs. Also be aware that, especially if the back tire is smaller, the ground clearance of the pedals will be reduced, so you will need to be extra careful while cornering. No need to downsize both tires (and in fact it would be a bad idea due to the brake issue).