Find FAQ: How to update my itunes library?

How to update my itunes library? --

I origianlly have 1200 songs in my home's itunes library, and I downloaded another 100 songs to my iPod from my friend's computer, while these 100 songs only exist on my iPod not on my home's computer.

So how can I upgrade my home's itunes library, then I can keep those 100 songs when sync with my home's itunes?

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How to update my itunes library?