HELP. I have a 5s How do I transfer purchased items to iTunes library so I can update OS?

HELP! I have a 5s using iOS 8.0.2. I want to upgrade to 8.1.2, which is available. However I get the message that there are purchased items on my phone that need to be transferred to my iTunes library. Can anybody tell me how to go about identifying which items these are and how to transfer to my iTunes library. Thanks so much!