Q: I want to update my iphone 5 to IOS ver 8 but itunes advises me to transfer some 'purchased items' on my phone to the itunes libary before I continue.

I'm using an Iphone 5 and need to upgrade to IOS Ver 8. As there's not enough space left on the phone I need to do this via iTunes on my laptop.

However, as I start the updating process, a dialogue box appears telling me that I should transfer some purchased items on my phone to the iTunes library before I continue.

Thus my questions...

How do I know what to transfer?

And How do I do the transfer?

Thanks in advance.