To access the Customizer via the WordPress Admin Panel, click on Appearance and then Customize.
To access the Customizer from the front end of your site, again, when logged in as an administrator, click on the Customize link in the WordPress Admin Bar.
Once the Customizer has loaded you will see a Sidebar on the left with some tabs, click Atlas Theme Options > Footer Options.
The footer also has 3 Menu Locations which can be managed via the Menus Editor that WordPress provides. See how to create and assign a menu.
The footer with all options configured will look something like this.
The Footer Content is editable via the Customizer (outlined above) and the Footer Menus are created/edited and assigned via the Menus Editor.
To change the Footer colors, you will need to edit the Atlas Fonts and Colors.