Q: How do I transfer purchased items on my iPhone 6 to my iTunes library?

How do I transfer purchased items on my iPhone 6 to my iTunes library?  I get this message when I want to update my OS software.

There are purchased items on the iPhone “Nick Leibold’s iPhone” that have not been transferred to your iTunes library. You should transfer these items to your iTunes library before updating this iPhone. Are you sure you want to continue?