1. Plug your iPhone 6 (Plus) into your computer with a USB cord. Your computer will automatically recognize your iPhone and will launch iTunes.
2. Click on the iPhone logo under "Devices" on the left-hand side of the screen in iTunes.
3. Click on the "Music" tab.
4.Choose "Entire Music Library" to put all of your music from iTunes to your iPhone 6 (Plus); Choose "Selected Playlists, Artists, and Genres" to add certain artists or playlists to your iPhone 6 (Plus).
5. Click on the "Sync" button.
however, it may erase your existing music while syncing, you can try some iTunes alternative like AnyTrans, copyTrans, etc.