There's instructions at this site that I printed out when I replaced one of my springs. I sacrificed an old screwdriver for the job (bent it with hammer and vise persuasion.) And stuff an old towel in the space below the hinge to "catch" the spring when it falls the first couple of tries to protect the paint. Good luck!
Good tip about the towel. When mine popped out it did it with enough force to take a small chunk of the fiberglass out of the edge of my door. I think my hinge area must be screwed up as I've reinstalled it twice and each time it eventually works its way out.
Before replacing, be sure to free up and lubricate the little toothed wheel that rides on the spring. Otherwise you'll be doing it all over again within a year. Skinny hands and long fingers help. Get help if you have ham-hock hands!
After trying for a half a hour to install the damm thing, I called my younger brother to the house and challenged him to beat my old installation time of five mins. (I lied) He did it in two minutes. Spring in and he is happy. Older brothers are smarter.
Don't waste the $$ on the tool. I have an extra one I'll loan you. We're pretty close to each other. Send me a Private Message with your address and I'll mail it to you. Send it back when you're done. Heck, it's just a bend Flat-Head screwdriver for $12. What a country.