When downloading the iOS app, you may be prompted to enter your Apple ID password. If you don't really download apps from the Apps Store, it can become easy to forget your password. You will have to enter your password before you can continue with downloading the app. If you need assistance with resetting your Apple ID password, you can use this link here; Reset Apple ID Password.
If you prefer to speak with an Apple support agent, you can go to this site: Contact Apple Support. Here, you will fill out a form and someone from Apple support will contact you to reset your password.