how do i change my password ?

If you know your current password then you can log into your account on your computer's iTunes via the Store > View Account menu option and on your account's page click on the Edit button to the right of your account id. Or on an iPad, iPhone or iPod Touch you can tap on your account in Settings > iTunes & App Store and log into your account and do it there.

If you don't know your current password then http://iforgot.apple.com