What do you do when a guy asked you out and you said yes but now you changed your mind?

I have no idea why you have changed your mind, but the only excuse you should have for not being responsible for a promise of dating this young man is he isn't a good person and you're afraid of him. If he's a nice guy give him a chance and you may just find out you like him. You gave your word and you'd better start learning now that your word builds character. If he's OK, you aren't afraid of him, then go out with him and don't go on another date with him if you don't care enough for him.