Social Media link colors

In My page, the header has a dark colour so facebook icon isn’t very visible. how can I change colour of “F” in white or the round point?
thank you for your help.

Somewhere I saw this comment: “The images for the Facebook social media icons are located in the images folder and are called facebook.png and facebook_2x.png. You can edit them manually.”

Ok… but, where is that folder? (can I upload freely to it?
Please explain step by step how to rewach it 🙂

Thank you!!!