Have researched this and no fixes have helped. I have songs on my iPhone 6+ that I do not want on it, and I have completely deleted those songs from my iTunes music library. So when I then sync iTunes to my iPhone, those unwanted songs still remain on my iPhone. Shouldn't the syncing effectively "overwrite" the library that's on my iPhone? (this is also happening with my iPad) Appreciate suggestions - thanks!
Syncing doesn't actually overwrite the iPhone music library, for whatever reason. Try turning Music sync off entirely (plug-in iPhone, select it in iTunes, go to the Music tab and uncheck the main box), sync, and then turn it back on and sync again. This should cause all of the unwanted songs to be removed.