How to Reset the Password for the App Store on My iPhone
Anyone can access the iTunes App Store to browse media items and read product reviews. However, if you want to purchase and download content from the App Store, you have to sign in to your account with your Apple ID and password. Apple ID passwords help ensure unauthorized users can't access your account. If you think your password has been compromised, or if you've forgotten your password, reset it using your Apple ID email address.
Navigate to "appleid.apple.com" in your Web browser.
Select the "Reset Your Password" option under the "Already Have an Apple ID?" heading. A new page loads.
Enter your Apple ID into the "Apple ID" field. Click the "Next" button.
Select the "Email Authentication" option to authenticate your account via email.
Click the "Next" button. A new page loads notifying you that the email has been sent.
Navigate to your email account if you choose the "Email Authentication" method. Open the "How to Reset Your Apple ID Password" email from Apple.
Read over the information provided in the message, then click the "Reset Your Apple ID Password" link. A "Create a New Password" webpage loads.
Enter your new password, then click the "Reset Password" button to complete the procedure.