Q: latest version of safari

When you update your system, Safari is also updated. So, you 6.1.6 is the latestfor your system. To update Safari further, you need to upgrade your system.

The latest is 9.1.1 in El Capitan.

As macjack mentions you will need to upgrade OS X in order to update Safari.

See if your Mac meets El Capitan requirements here >  Upgrade to OS X El Capitan

El Capitan is free and available for download at the App Store.

If you do decide to upgrade, check to make sure your applications are compatible.

Application Compatibility

Applications Compatibility (2)

El Capitan 10.11 Compatibility information

Do a backup before installing, preferable 2 backups on 2 different drives.

Additional to Eric's advice, also check that your printer and scanner devices have vendor drivers for El Capitan. Some don't and you will be annoyed if you don't check first.