how do i transfer music from my mac to my iphone 6s

Install iTunes version 12.3.2 on your computer if it isn't there already (https://itunes.com) (Your computer must have OS X 10.8.5 or later). Connect your phone to a USB port on the Mac. Click on its icon in the top navigation bar in iTunes. You may have to enter your iPhone's passcode the first time, and click to "trust" the computer and the iPhone.

You now have a couple of choices. One is to check "Manually manage music and videos" in iTunes, then click on "Music" near the top of the left navigation panel in iTunes. Drag the music you want on the phone to the Music entry below, under "On My Device". This is not what I recommend, however.

The alternative is to uncheck "Manually manage.." then click on Music near the top of the left Navigation pane. Check either "Entire music library" or "Selected playlists..." and check the music you want on the phone, then click Sync.

iTunes 12.3.2 requires Windows 7 or later. and iOS 9 requires 12.3 or later. See this for additional information: If iTunes doesn't recognize your iPhone, iPad, or iPod - Apple Support