Have you tried to use the Add or Remove programs functionality in Windows on that Instance?
It should work anyway.
Have a nice day?
I'd recommend you log this as a support call. Depending on the history of how you got to 7.4, there were some issues with the installer that created the effect you see. It is possible to edit the config file the instance installer uses to make it forget an instance so you can start again, but that might actually make things worse! Best to call on your local support team