If possible, it is advisable to update signatures from within the antivirus. You can find this option in the tray context menu and it ensures that the antivirus program compares the scan results to an updated signature database so that it detects both recent and known threats. However, if not possible, you would have to manually update your program using antivirus definitions. And to do that, you would have to save the installer to your PC.
Definitions are free and compatible with all Windows operating systems.