How to replace rear wheel of k2 attack 20 bicycle

Unless you have a deep sentimental connection with the bike, it may not be with it:
Rear wheel - $35-$45
Rim strip/tape - $1
Tube - $8-14
Tire - $10-20
Gear cluster - $25-40

New Huffy is about $150-250

You might be able to find a used bike or wheel at a garage sale that would work.

you can't tell without looking to see what is on the bike but if it is a thread-less stem you can try taking handlebar off an then take stem off head tube and turn it a 180 degrees, the two are connecting the opposite things. that should raise the angle of the stem and bring handlebar up. that is a free solution.

from the frame or the pads? if it fell from the frame then you might need a new nut or maybe see a bicycle repair shop.

You could Google that. "SRAM S30 sprint rear wheel spokes" or call a shop that sells SRAM wheels.

Don't replace the wheel, get it fixed. The spoke can be tightened and the rim tensioned and trued. The rim strip probably needs to be looked at for exposing the spoke ends to the tube.

Any Bike Shop.

You need to replace your freewheel (about $20). I'd advise taking rear wheel to a repair shop, as you'll need a freewheel removal tool (they retail at about $25) and most shops will replace it for free (with their tool) if you buy the part there.
If you pick up the tool, here's the instructions:

http://bicycletutor.com/replace-freewheel/

Could have sticky or seized up crank bearings where the pedals are or the rear wheel bearings. Have you tried pedaling and spraying a lubricant where all the bearings are or remove and re-grease the bearings?

Here is the perfect site for you:http://www.bikeguide.org/how-to/brakes/brakes.php They're called gyro brakes by the way.

Stop everything! Do not take the shifter apart! Go to the rear derailleur and look where the cable is attached. There is a bolt with an allen key head, it is an 8mm. Losen off the bolt. REFER TO PARKTOOL.COM