There is a firmware change log here (S60 Inside).
You can check the current version of your firmware by entering *#0000# into the dialer.
Additionally if you have come across the 'expired certificate' bug, which prevents you installing new software and were unable to fix the issue by resetting your device, then this firmware update should fix the problem. You will need to update the firmware and then reset the device via the *#7370# code (entered into the dialer) along with default lock code '12345'.
Nokia Software Update is a component of PC Suite.