How to transfer only 1 app to my new iphone?

So I got an iPhone 6S and have been using it for a while but there's just 1 music app from my old phone that I want to transfer to my 6S. Is there a way to only backup 1 app from my old phone without overwriting all the apps on my 6S…

You can do that by making sure you only transfer one app. Don't transfer 2 cuz that's more than 1.

Using iTunes you can back up and restore just one app. Do a MANUAL backup, and just backup the 1 app, then do a manual restore and restore just the one app to the new phone, or just redownload the app from iTunes.

Just download that particular app when you receive your iPhone and I would also recommend you to install Leo Privacy Guard so as to keep your Android or iOS mobile totally safe.