Reinstall Os X Yosemite

This is a hidden partition on your boot drive introduced with OS X 10.7 Lion, so the technique I'll describe works with OS X 10.7 Lion, 10.8 Mountain Lion, OS X 10.9 Mavericks and OS X 10.10 Yosemite. Resetting Mac OS X Yosemite to its original state. When you're ready to take the plunge. Quit out of the Disk Utility app, and this time choose Reinstall OS X and Continue.

Start up from macOS Recovery

Determine whether you're using a Mac with Apple silicon, then follow the appropriate steps:

Apple silicon

Turn on your Mac and continue to press and hold the power button until you see the startup options window. Click the gear icon labeled Options, then click Continue.

Intel processor

Make sure that your Mac has a connection to the internet. Then turn on your Mac and immediately press and hold Command (⌘)-R until you see an Apple logo or other image.

If you're asked to select a user you know the password for, select the user, click Next, then enter their administrator password.

Reinstall macOS

Select Reinstall macOS from the utilities window in macOS Recovery, then click Continue and follow the onscreen instructions.

Follow these guidelines during installation:

If the installer asks to unlock your disk, enter the password you use to log in to your Mac.
If the installer doesn't see your disk, or it says that it can't install on your computer or volume, you might need to erase your disk first.
If the installer offers you the choice between installing on Macintosh HD or Macintosh HD - Data, choose Macintosh HD.
Allow installation to complete without putting your Mac to sleep or closing its lid. Your Mac might restart and show a progress bar several times, and the screen might be empty for minutes at a time.

After installation is complete, your Mac might restart to a setup assistant. If you're selling, trading in, or giving away your Mac, press Command-Q to quit the assistant without completing setup. Then click Shut Down. When the new owner starts up the Mac, they can use their own information to complete setup.

Other macOS installation options

When you install macOS from Recovery, you get the current version of the most recently installed macOS, with some exceptions:

On an Intel-based Mac: If you use Shift-Option-Command-R during startup, you're offered the macOS that came with your Mac, or the closest version still available. If you use Option-Command-R during startup, in most cases you're offered the latest macOS that is compatible with your Mac. Otherwise you're offered the macOS that came with your Mac, or the closest version still available.
If the Mac logic board was just replaced, you may be offered only the latest macOS that is compatible with your Mac. If you just erased your entire startup disk, you may be offered only the macOS that came with your Mac, or the closest version still available.

You can also use these methods to install macOS, if the macOS is compatible with your Mac:

Use the App Store to download and install the latest macOS.
Use the App Store or a web browser to download and install an earlier macOS.
Use a USB flash drive or other secondary volume to create a bootable installer.

If you are a fan of the older OS X 10.9 Mavericks over the latest OS X Yosemite version and wish to revert to the previous version from the latest OS X 10.10 Yosemite, you can do that via the downgrade process.

The following tutorial explains an easy method to downgrade from OS X Yosemite to the Mavericks version. But make sure that you have backed up all your data before following the downgrade process.

IBTimes UK will not be responsible for any damage during the downgrade process.

How to downgrade OS X Yosemite to Mavericks

Downgrading to OS X Mavericks is an easy method. You need to install a Time Machine Backup of your Mac on OS X Mavericks
Also copy and save all your files from OS X Yosemite as you cannot restore a Yosemite Time Machine backup once you switch back to Mavericks.
With the Time machine drive plugged in or available on the network perform the following:
Click on the Apple in the upper left and choose Restart
When Mac is restarting press the Command and R keys together until the screen turns grey
On the next screen choose Restore from a Time MachineBackup and click Continue
Click on Time Machine disk then choose the backup you want to restore. Ensure this one is with Mavericks
Once you have finished the backup, follow the on-screen prompts to complete the installation and restore from backup
Note: Without backup you can still downgrade but you need a USB drive and you can manually back up all your data. But this is likely to take a little longer.
You need to create a clean install of OS X Mavericks and then move your files back to the computer. But be careful while doing so as updates like these can change file structures and libraries used in apps.
Download OS X Mavericks from Mac App Store
Open the Mac App Store, click on Purchase then find OS X Mavericks, click Download then click Continue and wait for the download to complete
Click Install on the pop up that appears. Ensure that you have backed up all data as the following step will delete your data

Step-1: Connect the USB drive and open Disk Utility

Step-2: Select USB drive in the sidebar and click on the Erase tab

Step-3: Select Mac OS Extended (Journaled) in the Format box and name it as Untitled

Step-4: Click Erase and wait for the format to get completed

Step-5: When the format is completed, close Disk Utility and open Terminal. Enter the following command and press Enter

sudo /Applications/Install OS X Mavericks.app/Contents/Resources/createinstallmedia –volume /Volumes/Untitled –applicationpath /Applications/Install OS X Mavericks.app –nointeraction

Now you should wipe your hard drive and install OS X Mavericks. Click here for help. Now you should see OS X Mavericks running on your system instead of Yosemite.

Reinstall Os X Yosemite Macbook Pro

Recommend: If you are user of Windows PC, you can use Syncios to free backup old iPhone/iPad/iPod before iOS upgrading or downgrading. You can also backup data from old iOS and then transfer to new iPhone 6, iPad Aire or iPad Mini 3. What’s more,there are Huge and growing collection of free games, apps, wallpapers, ringtones for new iPad Mini 3 and iPad Air 2.Just simply download and have a try.

Os X 10.10 Yosemite Download

Windows XP, Vista, 7, 8

Mac Os X Recovery Os

Reference: GottaBeMobile
You can follow Syncios on Twitter, Facebookfor more tips and tricks.