If you can't enter your account because you've lost it, read how to reset it here.
If you just like to change your password to something else, you can simply follow these steps:
1. login to your account
2. click on your username in the upper right corner
3. Select "My Account"
4. select "Edit Password" in the menu on the left
5. change it to something nice, like "IL0v3QuizW0rks" :D