Change social icon

Hi, thank you for your amazing and flexible theme.
I’ve already created a child theme and add some styles to it.

One thing that I really want to change is the social icon of YouTube.
I want to change from “fa fa-youtube” to “fa fa-youtube-play.”
However, I could not find where is the file that I could modify.

Could you tell me where I could change the icon?
Thank you.

Sincerely,
Nora

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