Log into your account with your email address and password, then navigate to My Account at the top of the left-hand menu. Click Edit Account Details.
Here, select 'Yes' where it asks "Would you like to change your password?" Change your password to whatever you'd like (minimum of 8 characters).
Be sure to click the blue Save Account Details button at the bottom of the page to save the password change.
Have you forgotten your password? Read this article on how to reset it.