Help beside my iphone PLEASE?

Please help. How do I "log out" of one account on my iphone and put my account details contained by so i can then purchase things off the app store. Do I do this on the phone or the itunes on my computer. I am plugged into it now but cant see anywhere to do it. Thanks heaps. Also I want to "sync" something that I enjoy bought and it says it will erase every other App and its details if I sync it so Im too scared to.

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Answers:

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when you plug iphone into computer, open itunes and you click on iphone statue on left side of screen, does it come up with huge picture of iphone? If it does, on this screen it should have a checkbox saying 'manually order ....' Sorry can't remember rest of what it says. check this. that means you don't have to sync every time you want to include something. Drag the app onto iphone now and it should install without having to sync.

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