At first you might panic. Especially if this person is someone you see and interact with everyday, such as a coworker or classmate. If it's a distant relative or someone you see less than once a year, my first piece of advice to you is don't sweat it. If they're out of sight, they are out of your mind. And really, they're not worthy of even being on your mind.
So back to the people you see everyday. How could they dislike you? You're awesome (because you're reading this, duh)! But for whatever reason, (most likely because they have a pointy stick up their asshole), they don't like you. You might feel inclined to examine what traits you have that irritates them, and try to correct them in order to appear more likable. DO NOT DO THIS! You will come off as a desperate tryhard, and I will personally lose all of my respect for you. Have you ever seen someone try really hard to make a good impression? Was it painful to watch? That's right, it was. You will look just like that person except worse, because you'd be making a bad impression even worse. So yeah, don't change yourself just to please some douchebag. Besides, a true friend would tell you what personality traits you need to work on, someone who dislikes you would not. Plus! If you ~*~stay true to urself~*~ there is a chance that they will learn to like you!
Now you're probably like, "Sar-ah! That's so cliche!" Yeah, I know, but it makes sense, right? But you have another option, and that is to give them a reason to dislike you. Now, here's where you can be creative! I have disliked people for a plethora of reasons; they're too nice, they're too shy, they're self-centered, they're overly religious... as you can see, there are many reasons to dislike someone, and not all of them are justified. But that's the way humans are! We can't always justify the way we feel. That's why you should just embrace the fact that you talk too much or that you're a huge Nickelback fan or whatever and rub it in their stupid faces! They'll probably dislike you even more, but you're fresh out of fucks to give!
If all else fails, ignore them. They're not worth your time. Make it seem like you know something they don't. Sometimes you can't avoid them, so sadly you'll have to be pleasant and cordial even though you've probably fantasized about this great confrontation where you dramatically ask them to confess their reasons for hating you but you know what? You don't want to know. Chances are that you don't have a solid way to defend whatever quality you have that they don't like. You'll be like, "I'm only obnoxious because... um..." and it will be sadder than watching a puppy die. Well, maybe not that sad, but sad nonetheless.
And sometimes, there is a horrible circumstance in which the person who dislikes you is someone you have a crush on. It is so terrible
even though I've never experienced it! The above advice still applies, but if you're feeling particularly jilted, photoshop is a great way to console yourself! You can just take a picture of that doucher and turn it into something like this:
Just post that baby on the internet anonymously and hurray! You've ruined their life! But I should warn you that doing this would not be "taking the high road". Good luck!