When I sync my iPhone to another computer, what happens?

I really want to update my Iphone on another computer, but i'm scared to.

When i go to update it, it says "There are purchased items on your iphone that have not been transfered. You should transfer these."

HOW DO I TRANSFER MY STUFF ON TO THIS COMPUTER?

When you plug it into another computer that doesn't have any other devices synced with it just click transfer purchases. This will bring everything on your iPhone to the itunes.
Note- If another device is already synced with the computer it will ask you if you want to erase and sync or just transfer purchases. DO NOT CLICK ERASE AND SYNC! This will take all existing data on the itunes and put it on your Iphone.

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