If you have forgotten your password, on the welcome screen you will be prompted to "sign in", from there, click "Forgot Your Password?" and you will be taken to a new screen, where you can input your email address. An email will be sent to your address with a link to reset your password. If this is not opened within an hour, the link will expire. Make sure you try this when you are ready to reset!
If you know your password but would like to change it, go to "My Info" and "Account Info". From there, type in the new password that you would like to use. You will need to confirm this by entering it twice.