Changing RoboHelp File Association

TCS5 | Windows 8.1

My laptop's been rebuilt, so I've installed TCS5 and then re-installed TCS4. However, because I've done it in this order RoboHelp projects are defaulting to RH10 when opened, as opposed to RH11. I've tried changing the association through the usual "Open with" option, but it doesn't make any difference. Is there a way I can force projects to open in RH11 by default without having to uninstall everything and starting again?

Thanks

Jonathan