The default site footer can be changed using the Zen Site Settings. The footer appears below all page content, at the bottom of the browser window (to add a bottom section to the main page content, see Adding 'top' and 'bottom' sections).
Note that portions of the default footer (Powered by Atlassian Confluence and Zen Foundation) may be required by your license:
You can add a Site footer using the Zen Site Settings menu item in the Admin Console. In the Site Settings panel, click the the Footer option. This will be either a "click to create" option, or a "click to view" option, depending on whether a Footer exists. Both links will open the Footer:
Please read Broken Links and Images on Zen Pages to ensure your footers display properly.