If you want to change your username or email address associated with your account, go to the My Account settings page and click "Edit Account."
However, before making any changes official, you'll need to confirm them with your password.
In addition, you'll be able to change your password at any time by pressing "Change Password" in the My Account settings. Simply fill in your current and new passwords, then confirm the change by clicking the blue "Save" link in the bottom right corner.
Note: You have only two hours to change your username and avatar. So, if you've reached the hourly limit, you'll have to wait till the next hour to change your name.