How to make your existing app compatible to iPhone 6 and iPhone 6 Plus

When iPhone 6 and iPhone 6 plus, the existing apps typically have no promblem running on the large screens. However, they are just scaled and everying looks fuzzy. I personally owned an iOS app in App Store. Becuase I know the bad user experiecne brought by not optimizing the app for the new iPhone(s), I want to update my app as soon as possible.
Then I began to do some research on online to see what I need to do. Surprisingly, I found very few resources teaching you how to do that. So I just want to write how I did that.