How to Unsync the iPhone from iTunes Manually

Launch iTunes on your old computer and connect your iPhone using the USB cable. If your iPhone usually syncs with your computer over your Wi-Fi network, then ensure it is connected to iTunes. Your phone's name should appear in the iTunes menu bar in both connection cases.
Click on your phone's name in the iTunes menu bar.
Click the 'Summary' tab and ensure the check box next to 'Manually manage music and videos' is checked.
Click 'Apply' to enforce the new setting.
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