Let me get this straight, go to your esf folder, for example c:\half-life\esf, in that folder there hould be a folder called models, go there, and then go to the player models folder. There you'll see all kinds of names of the characters used in ESF, go the folder of the character or model you wish to replace, paste/place your new file there, but make sure, that the name of your new file is the same as the old one.