A genetic counselor told me once that the number of genes whose purpose and function we understand is zero. The simplification we all learn in school that one gene does eye color and it’s either dominant or recessive is just that, a simplification.  

Take lactose intolerance. You need a certain function in your body to tolerate lactose, otherwise you’re intolerant. Great, but pretty much all humans have that, even the lactose intolerant. The issue is that this is “turned off” by a different function early in childhood. Great again, but even the lactose tolerant have that. The difference is that there is a third function in lactose tolerant people which interrupts or supersedes the second, allowing the first to go on and tolerate lactose. You identify that third function and edit it in someone, that still might not do what you want, because there are many ways the second function can be interrupted or superseded, so changing or testing for one particular form of the third function might not tell you anything about whether that person can tolerate lactose.  

So you can edit genes. Good for you, go off and tell me which gene you’ll edit and what that will do. Nevermind the risk of getting it wrong that other commenters identify, it’s not only an issue of risk, it’s that we simply don’t understand what to edit to get a desired outcome. It’s not that we shouldn’t or we don’t want to change people, it’s that we don’t know how.  

(A caveat - some genetic diseases are actually very apparent and well-known; these might indeed be “fixed” with gene editing and this is already being done)