You use the mabi-unpacker to find the file you want to replace and look at the name, and then extract the file you want to replace it with and put it in the same folder path in your data folder as the file you are replaceing(your C:/nexon/mabinogi/data folder) then rename it as the file your are replaceing with the EXACT name of the other file.
(ex: find the pally files and look at their names. then you find the file for lets say an incubus. extract incubus and go to C:/nexon/mabinogi/data/gfx/char/paladin/mesh and put it there, if you dont have that path in your data folder make the folders your self with the right names. change the name of the incubus file to the name of the pally file u want it to become and your done.)