Set or Change Hostname (via the GUI)

• Open System Preferences
• Select Sharing
  Select the Edit button and enter your desired Local Hostname and click OK
  

Note:  If the Edit button is greyed out, click on the Lock Icon, authenticate with an admin account

Set or Change Hostname (via Terminal) - OS X Mavericks or newer

• Open Terminal by going to Applications, then Utilities, and select Terminal

• The the following: