If you are upgrading from an ancient version (like 6.3 or 7.0), here is what 7.3.1 brings to the table:
Choose the folder where you want the program installed (the default is usually C:\Program Files\Cisco Packet Tracer 7.3.1 Complete Setup: through the shortcut options and then click Once finished, check "Launch Cisco Packet Tracer" and click cisco packet tracer 7.3.1 download