The correct code is (fal = light, far = regular fas = solid, fab = brands).
You’ll need the Fontawesome installed either via CSS or JS in your header section:
You’ll need to open and modify the social-media-links.liquid file in the Code Editor, or something along those lines.
I believe it is originally - that’s the line of code you replace with the Font Awesome Alternative. - the finished code will look something like this: Instagram