That much caffeine can cause headaches and other problems. Good idea to quit! You're also getting way too much sugar.
I'd just quit it all at once. You may experience some withdrawal symptoms, though not everyone gets them.
I needed to quit coffee when I quit drinking, since it suddenly started to give me anxiety attacks. I tested it today and the anxiety still happens.
But I did quit for a while and didn't notice anything special. You get used not having caffeine pretty quickly.
After you've quit, you can always see if you can stick to a couple of cups a day.
And if you want something as a replacement, go for green tea! I love it. It has so little caffeine in it that I can drink 10 cups and have no anxiety. And it's very healthy as well!