iTunes will not let me uncheck all music in Library on my Mac

[Pixeltaker]

I've been trying to put new music on the iPhone 6s through iTunes. All I've had is trouble. Every song was checked, so I went up to Edit, and the uncheck all was grayed out. When I wanted to sync a particular song, which I converted (with a Mac wma to mp3 format converter) to mp3, I right clicked the song. There was an option to send to device. The device was my phone, so I clicked to send it. It never appear on my phone. A day or so later, I tried again, but this time I went up to the menu bar and clicked sync to iPhone. That time it went over to my phone. Very confusing!

So how do I uncheck all the music on my Mac without going through the hundreds of songs I have and uncheck them manually?