If you don't know your password, you can reset it using the "Forgot Password" link on the login page of the website or the app.
If you do know your password, but would like to change it, you can do so from the "My Account" or "My Profile" page of your account when logged in through a web browser. To do so, first select My Account from the upper right hand corner. Next, select General Settings, then click on Change Password. You will then be prompted to enter a new password.