1) Click on the Apple icon in the upper left hand corner, then click on System Preferences.
2) In the Internet & Network section double-click on the Network icon.
3) On the left hand side select the Connection that is being used, and then click on the Advanced button.
4) Click on the DNS tab and use the – (minus) button in the lower left hand corner to remove bad DNS entries. Then use the + (plus) button to add correct DNS entries.
5) Click OK, and on the next screen click Apply if it is accessible. Close the Network window and Save Changes if asked. Close the System Preferences window, and you should be done.