How can I change password my Apple ID?

If you've forgotten your current password then http://iforgot.apple.com

If you know your existing password then you can change by logging it the account by tapping on the id in Settings > iTunes & App Store on an iOS device and then tapping 'view Apple id' on the popup, or by logging into it via the Store > View Account menu option on your computer's iTunes or via http://appleid.apple.com