Step 1: You must depressurise the keg first. Depressurize the keg by pushing down on the ball in the center of the valve. This can get messy, especially if there is any beer in the keg, and it takes a bit of pressure so I did not take a pic.
Lie the keg on its side and point the valve away from anything important, like your face, or the house.
Step 2: Using a screwdriver thats had a little work done on it by a bench grinder, hammer it in the little opening of the spring and pry the spring upwards. Ignore the big glaring warning on the top of the keg about removing the valve.
Step 3: Get the screwdriver up in under the ring and follow it around and remove the ring. Notice the end of the spring and how the screwdriver head is shaped. Both are tapered.
Step 4: Notice the valve is in its locked position. To remove the valve it will be necessary to beat on it with a hammer to move it to the unlocked position.
Step 5: Lift up the valve. It should easily come loose with just the tip of your finger, and will come loose with great force on its own if you forgot to depressurize the keg first!
Thats it. With a little bit of practice you can have one loose in less than a minute with nothing more than a screwdriver or hammer.