@francesco : First you will need to create a Social Menu containing social links. To know more about creating a social menu please follow the link below :
https://devotepress.com/tutorials-hacks/create-custom-menus-wordpress/
After creating the menu. Go to Appearance => Customize => Menus.
under Menu Locations click on View All Locations. Select the recently created menu under Social Menu at Left Side Above Primary Menu and click on publish.
Hope this helps you out.
Regards,
Tikaram