The easiest solution is to use a smaller hook. You might also need thinner yarn so your booties aren't too stiff/thick.
If you don't have a smaller hook, you'll have to change the pattern. It's hard to tell you exactly how without having crocheted the pattern because a bootie has a fairly complicated shape. One strategy you can try is to make one bootie as written to get a better idea of the construction. It would be a fast job since it's a small item, and once you've completed it you might find you'll have a better idea of how to make it smaller.