How do I upgrade safari bowser to the latest version? Some of the features are not available under the current version (12.1.2).

To upgrade Safari, you need to upgrade macOS on your MacBook Air. See information below. If you cannot upgrade to a current macOS, I would recommend, for security reasons, that you use a 3rd party internet browser that supports running their current version. The current version of Chrome, for example, will run on macOS 10.13 or later.

The model year of a MacBook Air is available at Apple menu > About This Mac.

A mid 2009 MacBook Air can be upgraded to El Capitan 10.11.6 but no later version. See: How to download macOS - Apple Support

A late 2010 through mid 2011 MacBook Air can be upgraded to High Sierra 10.13.6 but no later version. See: How to download macOS - Apple Support

A mid 2012 MacBook Air can be upgraded to Catalina 10.15.7 but no later. See: How to download macOS - Apple Support

A mid 2013 and early 2014 MacBook Air can be upgraded to the latest version of Big Sur macOS 11 but no later. See: How to download macOS - Apple Support

An early 2015 and a 2017 MacBook Air can be upgraded to the latest version of Monterey macOS 12. See: How to download macOS - Apple Support

A 2018 or later MacBook Air can be upgraded to the current version of Ventura macOS 13. See: How to upgrade to macOS Ventura - Apple Support