Choose Apple menu > Restart. Hold down the Command (⌘) and R keys as your computer restarts.
When you see a white screen with an Apple logo in the middle, you can release the keys.
Click Disk Utility, and then click Continue.
In the list at the left, select the item you want to repair.Macintosh HD.
Click First Aid.
Click Repair Disk.
If Disk Utility reports that the disk appears to be OK or has been repaired, you’re done.