How to change skin of units with AGE?

I need to change the Flying Dutchman skin to Juggernaut in Return of Rome, but I don’t know how to do it, I only know that I can’t do it without a datamod and AGE.
I’m a very beginner modder (this will be my first mod), so I need to explain in detail, where to change what, where to save, etc.

UPD: If I said something wrong, then I just don’t speak English (google translator will help), but for this game the non-English speaking community is so-so (for comparison, on Age of Empires Wiki in Russian there are 132 pages, but in English there are as many as 6650!)