You could begin by taking a personality test. There are many available on the Internet, or you can buy a book. Else you can take the time to reflect on who you are.
Your ability to change your personality is one for debate. If you believe that genetics plays a large role in your personality development or that personality is developed during childhood and is now fixed, then the potential for change will be limited. However, what is important is that you embrace all elements of your personality. After all, you can at least change your attitude towards your personality. Think of how your different traits are strengths to you and then use them in positive ways.
If however, you believe that your personality can go on changing throughout life, then think about the changes that you would like to make. However, you should change what you want to change, rather than feeling compelled to change. Change can also be about stepping out of your comfort zone. As such, you may do things that you believe you simply cannot do. Having completed them however, such actions will be assimilated into who you are and as a consequence, your personality has undergone a subtle shift.
Which is correct? How you perceive yourself, or how others perceive you? Watch an episode of Big Brother and you are likely to see a person shocked at how other people in the house perceive them. There is a difference between how they regard themselves and how they are perceived.
Think about your own life. Have you ever been told something about yourself that surprised you? Did you reject it or reflect on it? Sometimes we may underestimate who we are, or reject qualities we do not want a part of. If the disparity seems quite frequent, maybe it is time to reflect on who you are and acknowledge all aspects of yourself.