To install updates for Microsoft Office on a Mac (any version of OSX):

1. Open one of the Office applications (Word, Excel, etc.)
2. Click Help > Check for Updates.
3. Microsoft AutoUpdate will launch. Click Check for Updates.
4. Any available updates will be listed. Install them as necessary.

Note: The computer will check for updates automatically; you can update the schedule or switch to manual if necessary.

Source and more information: Microsoft's "Update Office for Mac automatically" documentation.