Unzip the contents into your Stardew Valley root directory and allow it to replace the files. If downloaded through Steam, that directory will probably be located at something like C:\Program Files\Steam\steamapps\common\Stardew Valley.

As always, it's a very good idea to back up the original files first. In this case those files are:

Stardew Valley\Content\Characters\Elliott.xnb
Stardew Valley\Content\Portraits\Elliot.xnb