i have an imac with os x 10.5.8 and want to upgrade. how do i upgrade to 10.6.8 or higher?

Click here, check that your Mac meets the requirements, buy and install the DVD, and then run Software Update.

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If you are looking to upgrade so you can sync or register an iPhone or other Apple device, you only need to upgrade to OS X 10.6.8 so you can install iTunes 10.7.