I don't like the app library. How do I change. back to the original home screen format

The App library cannot be removed. You don't ever have to use or see it. Place your apps however you want. The App library will always be the last page.

See this link on how to move and arrange apps --> Move apps and create folders on your iPhone, iPad or iPod touch – Apple Support (UK)

You can't. Just place the apps on your home screens and then ignore the library

Your Home Screen can be whatever you make it. The App Library will be the screen all the way to the right