You can change the default social icons by clicking on the icons from the signature preview on the right and then selecting which social icon you want to display from the drop-down list.
You also have the option to select “None” if you don’t want to display a social icon or “Custom Social Image” to upload your own icon.
To add an additional social icon to your signature, navigate to the Layout tab and click the small + symbol in any column you wish to add the icon into.
You will be able to create a new social icon field by selecting "New Custom Field" in the dropdown and giving it a label such as "Icon 8".
Once you're done, you can head over to the Details tab, and find the social icon you just added in the left-hand side and customize the icon to your liking.