A Favicon is a small image that resides in your browser’s tabs (and often bookmark lists), allowing you to identify the page - and the brand that it represents - at a glance.
Within your Shopify admin, go to Sales Channels > Online Store > Themes.
Click your theme, and select Customize.
On the left-hand menu, select Theme settings.
Scroll down to select Favicon.
To add an image as your favicon, go to Favicon image and click Select image. Upload a new image, or select a pre-existing image from your Library or Shopify's Free Images.