How to change Dynamics portal Administrator user?

Hello,

Does any one know how to change Dynamics portal Administrator User? I tried using below screen but "Ok" (tick button at the bottom) is not enabled until I changed other settings available on this form.

I have doubt it will wipe all my configuration and portal settings that I have made. I think this is GUI problem. Or does anyone know how to change portal administrator user?

Thanks,

Ashish