If you have a Late 2008 or newer MacBook Air, you can run OS X Mavericks. Open > About this Mac, and check which OS X version you have.
If it's 10.6.8 or later, open the Mac App Store and download OS X Mavericks for free. The Mac App Store won't let you download it if your computer is not compatible. Make a backup of your files with Time Machine and check that your apps are compatible > http://www.roaringapps.com