This'll be over in a few moments.
Note bene: Always save the original file just in case you don't like the mod! 

1) Take the downloaded .xnb file and put it into this Stardew Valley directory:
   ie. C:\Program Files (x86)\Steam\steamapps\common\Stardew Valley\Content\Portraits
2) Overwrite and enjoy!


Removal:

Replace the .xnb file with the original .xnb, or another modder's lovely portrait mod. :)