Safari is updated via iTunes. It lets you know this when you switch on your computer and open iTunes. It'll then tell you updates are available safari being one of them. Some people have the pop up option to tell you updates are available some have it when you open iTunes it then tells you. Just incase replies come back saying iTunes doesn't tell me. Only once you've updated and synced your phone will the newer version of safari be on there.
If you want to know what version you are using. Open iTunes and right click safari.
iHolophyte