If you have purchased your subscription or single issue via the iTunes App Store, signing into the app is not necessary and you will not have nor need an app user name or password. Your purchases should automatically be displayed via your iTunes Account Log on. For purchase troubleshooting please see here
If you received sign in credentials from the publisher, change the password provided by:
- Select the menu icon > go to "settings"
- Select "Account Settings"
- Tap on "Change Your Password"
- Update your details and tap "submit"
Note: The option to change the password is not available in all publications.