how to change my password

Your password for what ?

If your iTunes account then you can change it by logging into your account via the Store > View Account menu option on your computer's iTunes, tapping on it in Settings > iTunes & App Store on an iOS device and logging into it, or via http://appleid.apple.com - you might then need to log out and back into the account on your devices. If you've forgotten your current password then you can try getting it reset via http://iforgot.apple.com

If you forget your Apple ID password, follow these steps to reset your password and regain access to the Apple features and services that use Apple ID.

If you know the answers to your security questions

1. Go to My Apple ID (appleid.apple.com).
2. Click "Reset your password."
3. Enter your Apple ID and click Next.
4. Select "Answer security questions" as your authentication method. Click Next.
5. Select the birth date associated with your Apple ID, then click Next to begin answering your security questions.
6. After answering your security questions, you will be asked to enter and confirm your new password. Click Reset Password when done.

If you have access to the email addresses associated with your Apple ID

1. Go to My Apple ID (appleid.apple.com).
2. Click "Reset your password."
3. Enter your Apple ID and click Next.
4. Select "Email authentication" as your authentication method. Click Next.
5. Apple will send a message to the primary email address or verified rescue email address associated with your Apple ID. Open the message and click "Reset Apple ID password".
6. When the My Apple ID page opens, enter and confirm your new password, then click Reset Password.

If you have two-step verification turned on

1. Go to My Apple ID (appleid.apple.com).
2. Click "Reset your password."
3. Enter your Apple ID and click Next.
4. Verify your identity: Enter the Recovery Key you were provided when setting up two-step verification, then choose a trusted device that will receive the verification code from Apple.
5. After entering the verification code, create your new password and click Reset Password.

You can find this instructions from this articles:

http://support.apple.com/kb/HT5787