Hey stop that ****in caps lock and start think self..
for someone with 12 posts you seem to think you run the place =D,
well..... you dont >_>.
now to answer the posters question without being a total assclown.
find your esf folder, should be in C:/program files/steam/email adress/half life/esf
find the models folder and look for the one of the character you want to replace
put the new model in there and rename it to match the existing one (or overwrite it),and voilla
ALOT of esf model downloads are already named properly right off the bat, so it isnt hard figuring out where they go.