Did you ever ran DXDIAG.EXE to check if your DirectX Installation is OK ?
Officially there is no way to uninstall DirectX11 because it's integrated into the system.
The recommendation would be a repair install as outlined here Repair Install
Unofficially it MIGHT
work if you go in the registry, navigate to
and delete the DirectX Folder"/Key
That way you SHOULD
be able to run the webinstall from the link, Mickey gave you.
Before doing that I would at least export the whole key as a back up.
I usually don't recommend messing around in the registry as I have no prove that it'll work like this.(It sounds ok but I've never actually done that)
So consider this the very last try before a clean install from scratch.