I'm presuming you mean changing your attitude for the better.
If you start off as, say, a negative person and you want to be more positive, you just have to slowly integrate actions and thoughts of the opposite of yourself into your life. What I mean by that is to start thinking and acting more positive.
Every time you sigh, scream, feel sad, or get angry (usually over completely irrational things - human nature) there are hours, days, years, you will never get back. I'm not saying you automatically have to become Ms. Happy. That can be tiresome for other people to endure - but you have to realize that you can either view a glass as either half full or half empty.
For me, I've always been a positive person. I think the way I project myself make me a more approachable person, because I'm confident, happy, and optimistic about things. If you need an attitude change, these are three traits you will learn to adapt. It all comes with time and experience.
Remember, the way you project yourself means the world.
Rock your time on this planet (cheesy, but true!) and always know there is no one quite like you.