How do i do plug-in upgrade and uninstall?

I installed the plugins using XPinstall commands...

-------------------------------------
 xpi = {'My Mozilla plugin':'MyPlugin.xpi'};  //  "xpi" zip file contians 
install.js to do the installation

 if (InstallTrigger.enabled() )
{
    InstallTrigger.install(xpi);
}
else
{
    alert("Software installation is not enabled");
}
----------------------------------------------

Installation worked fine.


Now I use install Trigger.getVersion to check the installed version...but 
this below code doesn't seems to work.... ANy idea?

existingVI = InstallTrigger.getVersion(http://sat_dev1/'MyPlugin.xpi');

 if (existingVI.compareTo("1.1") <= 0 )
{
    alert("version of the installed plugin <= 1.1");
}
else
{
    alert("version of the installed plugin > 1.1");
}

How do i force the plug-in upgrade?

Also, can i un-install the plug-in?