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Read the Restore Your System page and click Continue.Click Restore from Time Machine Backup from the OS X Utilities menu.Hold down Command+R while your Mac boots.
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If not, or if you choose not to, you can restore at any time. You should be asked if you want to restore from Time Machine as part of the reinstallation process. How to restore your data from Time Machine If that happens, complete the installation of Lion, Mountain Lion, Mavericks, or whichever version you're offered, and when you're done and rebooted, go to the Mac App Store and re-run the OS X Yosemite update. Note: Depending how old your Mac is, you may get an older version of the installer-typically the one that originally shipped on your Mac (That's especially true if you end up in Internet Recovery mode). When you're done, your Mac will reboot into OS X Yosemite. From this point on it's the same as a standard update, simply continue on and agree to the licensing terms as you go.
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