There's a readme.txt file inside the .zip archive that you downloaded explaining how to do it. Basically, just put the .vpk file in steamapps/common/Team Fortress 2/tf/custom. If you don't know how to find your steamapps folder, you can right click the game in Steam and go through Properties -> Local Files -> Browse... to get straight to the "Team Fortress 2" folder instead. Also keep in mind that model replacements like this one don't work on Valve servers nowadays, so you need to find a community server that doesn't use sv_pure or play offline in order to test it.