How to edit Footer area?

On Appearance > Customize > Header and Footer panel there is a Footer Page Slug. 

This field should contain the slug of the page which serves as a template for footer area.  All pages inherits settings, elements and shortcodes of the page referenced via this field.

In order to change elements in footer, please locate the page (on all pages list) with the slug entered in Footer Page Slug field and edit it.