Reinstalling OS X Mac Yosemite 2015

If you're having problems that you can't resolve them you may need to reinstall

the Mac OS X operating system 

here is how:

       You must have internet access.                

1- Macs running Yosemite have a  built-in recovery disk that you can use to reinstall Yosemite without loosing your files and your settings.

2- Just Go to the Apple menu and select > Restart.

3- As soon as you Mac restarts hold down the Command (⌘) and R keys at the same time. 

4- Then Select Reinstall OS X and then click Continue.

5- Follow the onscreen instructions,
In the pane that comes up where you select a disk, select your current Mac OS X disk.

Let it install when it restarts you will still have all your Files and setting with a brand new MAC OS.