this one is little tricky if it doesnt appear in your game without doing hard stuff so you will have to open the game VERSION launcher that formated as java file

open it as winRAR or any application like it then go to

assets

minecraft

texture

entity

iron golem

now copy paste the new iron golem PNG file to old one

after that open the iron golem PNG that are in the iron golem folder

if it appear as the new one so you are done if no contact me