If your license of Vectorworks is Designer with Renderworks, then you can simply replace the serial number.
If you only purchased Fundamentals or Landmark for example, you will need to uninstall Vectorworks and reinstall with your new serial number.
To enter this new serial number in your already installed copy of Vectorworks, go to Tools > Options > Vectorworks Preferences
Under the "Session" tab, choose "Serial Numbers"
Click the EVAL serial number and choose "Remove", afterwards choose "Add" and enter your new serial number. Press "Add" again when complete. You should see your new serial number and activated products listed to the right:
Restart Vectorworks, your copy will now be a professional and will no longer expire.
Edited by JimW