Syncing iPhone with new computer.

The other day my mac broke down and I had to take it to the apple store to get it fixed. They wound up having to replace the hard drive. I got it back today and I'm trying to update the software on my iPhone via iTunes but when I try to do it I get a window that says I should transfer the purchased items on my phone into my iTunes library before updating. I was just wondering how I can do that? I've tried syncing the phone but that didn't do the trick.
What can I do?

 

thanks so much for that. Absolute life saver