Hi photogal and welcome back to Mac-Forums.
I think that, with no offence intended, that you may be going about this the wrong way
Please forgive me if I go through what I think you want to do:
You want to sync your 6S with iTunes and only put on the iPhone what you choose?
If this is your aim -
Directly attach your 6S to your Mac via USB. Open iTunes if it does not do it automatically (NB you can choose whether iTunes opens and syncs as soon as you attach your iPhone or disallow this - I can tell you how if that matters to you.)
In iTunes, very top, far left, click on the iPhone icon (do not click on the iPhone under Devices).
This opens a Window with, on your right, "Check for Updates" and below, "Backup this Device" and so on.
Now, look to your left under the iPhone icon and you will see Summary, and under that, Apps, Music, Films, and more.
Click on Music.
When you do that, it opens a window with all your Playlists, Artists, Albums, Genres etc.
This is where you choose what you want to sync with your 6S. Then click on sync - very bottom (next to Done).
If that is your aim, problem solved.
Completely separate from that is the question of checking/unchecking songs in the iTunes library.
The way to do that is not via Edit. Instead, select all the songs you want to uncheck manually (or if you really mean to uncheck all of them, you can click on one song and then Command + A.)
Then Right Click and choose "Untick Selection". This will do just that - remove the tick from all those songs you have selected. But I doubt if this was your intention. I suspect all you wanted to do was choose which songs to sync with your 6S. Am I right?
Please post back.
Ian
