Summarize:

You will need to have access to the inside of your gun, so remove the screws on the stock and body of the gun. The screws on the stock are a different size than those on the rest of the gun, so make sure to keep them separate to make it easier when you put the gun back together. You will be working primarily with the part of the gun that contains the spring. This is the part that is made up of a thick cylinder connected to a thinner cylinder that runs along the middle portion of the gun. It is called the plunger tube.  In order to remove this piece, you will have to remove the internal screw located directly to the lower right corner of the larger cylinder of the plunger tube. Once you have removed this main piece, cut off the black rectangular piece of plastic that is connected loosely to the thinner cylinder. The wider cylinder is closed off at the end by a black piece of plastic. (This is the end of the piece around which the spring is coiled inside the cylinder; this will come up later.) Remove the screws that hold this piece on, and then pull out the long piece of plastic with the spring around it. Put this piece aside for now. When you look down the plunger tube, you should see another piece of plastic that blocks you from seeing the other end. The center part of this piece is held in place by 3 prongs. Using your pliers (you need long-necked pliers for this part), cut the three prongs. Turn the plunger tube upside-down so that the loose pieces fall out. Now you will be able to see through the whole tube, but there will still be a peg-shaped piece of plastic that partially blocks the tube. Put your screwdriver into the plunger tube so that the end of the screwdriver is located in the center of the peg. Use a hammer to hit the fat end of the screwdriver until you knock out the peg. Now you will be able to see completely through to the other end. Find the piece of plastic that holds that spring that you took out and put aside earlier. #*Unscrew the end so that you can remove the spring. Take off the spring and replace it with a more powerful spring. Then screw the end back on to hold the spring in place. Note that you can add multiple springs off different strengths, as shown here: http://www.youtube.com/watch?v=7tv2KvWjD0w. Screw in the screws that you unscrewed earlier, so that the cylinder looks just like it did earlier from the outside. Replace the plunger tube to its original position. There are a few small parts that you will need to line up, but this should not be a problem. Before closing the gun, make sure that the plunger tube lines up correctly with the barrel. Replace all the screws that you unscrewed earlier. Your gun should now be more powerful. Have fun, but be careful.
Unscrew the screws on your gun. Open up the gun. Remove the screws to open up the plunger tube. Remove the prongs. Knock out the peg. Switch out your spring. Put the spring piece back into the plunger tube. Put the plunger tube back into place. Close the gun. Fire away.