If you're not willing to take care of your pet, you simply should not have one. It's the same thing as taking a dog home from the pound and then locking it in a closet and only taking him out and cleaning the room once a week. It's animal abuse.
When you bring home an animal, you're making a promise to them to take care of them. This means providing them with a healthy environment, and that takes work. You have to commit to putting out a certain amount of effort--and it doesn't take much effort to do a simple water change. This is your responsibility--if you can't handle it you should take him back to the pet store.
100% changes are easy. Just put the fish back into his cup and pour the water in the tank into the sink. You can use a strainer so you don't let any gravel go down the drain. Rinse the tank and the gravel with hot water, then adjust the temperature of the tap water until it feels the same as the water in the fish's cup and fill up the tank. Add dechlorinator and wait for it to set in and for the nitrogen to dissipate. It doesn't take long. Then just float the fish's cup in the water. Every few minutes pour some of the water in the cup out, and put some of the new tank water in. After about 15-20 minutes the fish will be acclimated and ready to go back in his tank. This only takes about 5 minutes of real work, and the rest is just checking on him periodically through the acclimation. Easy.