find this mod's gma file, should named similiar as this mod, and unpack it with gmad.exe( if you dont know how, google it, this is basic skill for this game). After unpacking you should have 2 folder and 1 json file, put all these folders and file into another folder, name the folder whatever you like. (eg. you can have a folder nemed as "alyxreplace" and in the folder you need have 2 folder "materials" and "models", and 1 json file).
Then you just put the folder you just make into you game addon folder , should be somewhere like, [your game folder path]\SteamLibrary\steamapps\common\GarrysMod\garrysmod\addons

and restart you game, now you should see the model changed

sry for the inconnivent, source engine pathing for replacement models sometime is really wired.