How to change Apple ID on App Store?

How did that happen?

Where are you seeing this other Apple ID exactly?

There can only be one Apple ID signed in to the App Store, and its set up in Settings ➜ Apple ID ➜ Media & Purchases.

If you are seeing this Apple ID somewhere else, then please clarify where.

Have you forgotten your Apple ID?, Check your Apple ID via your email box, see this Apple article: If you forgot your Apple ID

If you forgot your Apple ID - Apple Support                  

OR see -> Having trouble signing in?

https://iforgot.apple.com/password/verify/appleid