How to Reinstall Lion or Later W/ or W/out Erasing the Drive
A. The Following will Re-Install OS X from Scratch
Make sure you have a current backup before proceeding.
Boot to the Recovery HD:
Restart the computer and after the chime press and hold down the COMMAND and R keys until the menu screen appears.
Erase the hard drive:
1. Select Disk Utility from the main menu and click on the Continue button.
2. After DU loads select your startup volume (usually Macintosh HD) from the
left side list. Click on the Erase tab in the DU main window.
3. Set the format type to Mac OS Extended (Journaled.) Optionally, click on
the Security button and set the Zero Data option to one-pass. Click on
the Erase button and wait until the process has completed.
4. Quit DU and return to the main menu.
Reinstall OS X: Select Reinstall OS X and click on the Install button.
Note: You will need an active Internet connection. I suggest using Ethernet if possible
because it is three times faster than wireless.
This should re-install the version of OS X that you had installed.
B. Reinstalling Lion through Yosemite Without Erasing the Drive
Make sure you have a current backup before proceeding.
Boot to the Recovery HD: Restart the computer and after the chime press and hold down the COMMAND and R keys until the menu screen appears.
Repair the Hard Drive and Permissions: Upon startup select Disk Utility from the main menu. Repair the Hard Drive and Permissions as follows.
When the recovery menu appears select Disk Utility and press the Continue button. After Disk Utility loads select the indented Macintosh HD entry from the the left side list. Click on the First Aid tab, then click on the Repair Disk button. If Disk Utility reports any errors that have been fixed, then re-run Repair Disk until no errors are reported. If no errors are reported click on the Repair Permissions button. Wait until the operation completes, then quit Disk Utility and return to the main menu.
Reinstall OS X: Select Reinstall OS X and click on the Continue button.
This should install the same version of OS X over the version of OS X you have installed.
Note: You will need an active Internet connection. I suggest using Ethernet if possible because it is three times faster than wireless and more reliable.
C. Reinstall El Capitan Without Erasing the Drive
Make sure you have a current backup before proceeding.
Boot to the Recovery HD:
Restart the computer and after the chime press and hold down the COMMAND and R keys until the menu screen appears.
Repair
When the recovery menu appears select Disk Utility and press the Continue button. After Disk Utility loads select the indented Macintosh HD entry from the the left side list. Click on the First Aid tab in the toolbar.. If Disk Utility reports any errors that have been fixed, then re-run First Aid until no errors are reported. Wait until the operation completes, then quit Disk Utility and return to the main menu. Select Restart from the Apple menu.
Reinstall OS X: Select Reinstall OS X and click on the Continue button.
This should install the same version of El Capitan over the installed version of El Capitan.
Note: You will need an active Internet connection. I suggest using Ethernet if possible because it is three times faster than wireless and more reliable.