Upgrading to Apple OS X Mountain Lion does introduces it's own set of annoyances, and one of them is auto system updates which does not allow system to power down until finished. With a slow internet speed and GB's of update to download this can be a real annoyance, as shown in the screenshot below the update would take more then a day to finish.

Luckily, there is a way out as demonstrated below.

Apple OS X Mountain Lion is now available for upgrade, there could be three major possible scenarios for an upgrade and here are they :

1. Your Mac was bought on or after June 11, 2012 - You will get the "Mountain Lion" upgrade for free, simply claim your free download coupon and download from Mac App Store.
2. You have OS X Snow Leopard/Lion installed and you bought your computer in late 2009 or after - upgrade for $20 through the Mac App Store.
3. Older computer - Check system requirements and upgrade to Snow Leopard or Lion and then procces to "Mountain Lion" upgrade.

Here are screenshots of OS X Lion to OS X Mountain Lion upgrade process.