How can I uninstall previous version windows application use install Shield script?

Question

I have an InstallShield project, and try to build setup.exe file, now I have an old version application installed in my PC, and I need to uninstall this old version application, then install the new version of this application, I use below script:

function UninstallOld()
begin                             
  LaunchAppAndWait("msiexec.exe", "/x {product GUID} /qn", LAAW_OPTION_WAIT);
  UninstallApplication("{product GUID}", " /x /s", LAAW_OPTION_WAIT);
end;

But it doesn't work, could anyone shed me some lights on this?

Thanks in advance.