Have no fear, it's a lot easier than it looks at first

.
Things might differ a bit if you have the "puck" style adapter without an upper heim joint (like Chris's) vs mine with an upper heim, but it amounts to the same thing.
Remove them from the truck.
Grab your adjusting wrench, the one with the pin that fits the adjusting collar on the shock body. This collar sits at one end of the spring. Find the set screw, it's a small allen key screw somewhere around the adjusting collar, and loosen it. Now set the body end in a soft jaw vice...I usually just put a couple pieces of wood over the jaws and call it good.
Back off the collar until the old spring is loose, and voila! You can put the new one on

. You'll obviously need to back the collar a little further to get the longer spring on, but it's that simple.
Something I've found very helpful over the years is gob a ton of grease on the threads before you start backing off the collar, and before you start tightening it down. That ensures you don't gall the aluminum collar on the steel body of the shock. Find the height you want and put the set screw back in and you're done!
BTW the more grease you can get between the collar and body, the more likely you'll be able to adjust the coilovers down the road. Being dissimilar metals, they will eventually corrode together if not greased. What I've found works well is stick some grease down the set screw hole, then run the screw down. Repeat...it will force grease between the threads where water could collect. Stop when you see clean grease forced out between the collar and the shock body.
Hope that helps!
-Sean