Wordpress Plugin Upgrade Hook Function

I'm developing new version of my wordpress plugin (http://wordpress.org/extend/plugins/facebook-send-like-button/) .

New options (add_option()) coming with new version. But i can't register this new options automatically.

For example there is fgb_single option in new version. Where should i put add_option('fgb_single', 'on'); code in my plugin's file?