Undo or reverse YouTube update on my Samsung Galaxy S4
If it's a system app, you should be able to uninstall updates by long pressing the icon, dragging it to "App Info", tapping the overflow menu at the top right, and tap the "uninstall updates option" 

Next, go to settings>security>unknown sources and check the box.

Finally, go to APK Mirror, search for YouTube, and download the 2nd most recent apk (choose ARM Varient) and open it. 

Enjoy the older YouTube version.