how to update mac os x 10.7.5 to os 10.9 or above on a mid 2011 mac air

Hi there, no sure how to update mac os x 10.7.5 to mac os x 10.9 or above as when I check for updates it displays your software is up to date.

Also having issues with safari as it won't open any page displays "safari can't find the server xyz..."