How To Change Your Apple ID/Password
Go to https://appleid.apple.com/ and select "Manage your Apple ID". Log in with your Apple ID and password, then select Password and Security from the left column. Your Apple ID password is used for most everything Apple-related, including your Developer Account, iTunes, the App Store, and this site. An iBook account requires an additional/different login. If you want to change logins for Team Members you can do so via iTunes Connect*.
Passwords are case-sensitive. At least one character must be upper case.
At some point Apple may force you to change your Apple ID password anyway, but do not do so in response to any unsolicited email by clicking on a link provided within it. Such "phishing" attempts are common. Change it only after attempting to log in here, and your login attempt is blocked due to an expired password or other Apple-imposed requirement.
Additional information at: Account Management - iOS Developer Program - Support - Apple& theApple ID FAQ.
And remember, devs should rotate/change their pwds on a routine basis, so take this opportunity to practice safe security.
*For iTunes Connect: Log in to iTunes Connect/Manage Users/...and then use 'Edit Profile' for each user and click on 'Change Password'. You will need to know the current password to make changes here.