No problem, you only need to download QGIS 2.4 and install it.
- the application (QGIS.app) is placed in /Applications/
- the frameworks are in /Library/Frameworks/ (/Library/Frameworks/GDAL.framework for example)
- the extensions are in /Users/you/.qgis2/python/plugins (hidden folder, /Users/me/.qgis2/python/plugins/tablemanager for example)
- the Python modules are in /Library/Python/2.7/site-packages (/Library/Python/2.7/site-packages/matplotlib for example)
You can upgrade 1) or 2) or 3) or 4) independently. (Mac Os X is an UNIX system)
If you want to keep the old 2.2 version, rename the application, QGIS_2.2.app, for example, otherwise QGIS.app is replaced by the new version.