Help on Altering a specific feature of a UI

[TFirth101]

Ok,
The other day i downloaded and installed the fetish nightfall ui. I love it, and think its great, before it i had a ui which was quite good, but i prefer a couple of windows in the old ui to the equivalent new ones.
What i wanted to know is how do I replace some of the current windows with some of the old windows i had. Do i just replace the correct file? If so, how do i know which file alters which window.
Tnx for any help.