The directory is C\sierra\Half-Life\esf\models\player\ by default.
First your name the old files with a "bak" in front of them, then
you must rename the downloaded models(the files AND the directory) to the old files, for example, when you download a new goku model, you must rename the "goku" file to "bakgoku" and put the new files into the directory named as "goku". But I will have the old ones forever, because the new models have sometimes bugs, and the old ones are better to play I think.