GLOBAL CONTENT

Global content is content shared in portions of a website.
Common examples are the header and footer of the site or the sidebars, for example, of a blog.

When editing a global component, a different editing experience is provided for content editors that makes it easy to edit the global content and preview changes on pages before publishing. When the change to the global content is published, all pages and/or the blog, which contain that global content, will be updated. 
ATTENTION: generally themes have 2 different header and 2 footer for website page and landing page and should be edited separately:

To edit a global content, click with the mouse on the "zone" where the global content you want to edit is located; for example if you want to edit the header, click with the mouse over the site menu (which is located in the header) a popup will open (see image on the right).

Click the button and edit the global content. Generally for the header you can edit the logo and the menu. At the end you have to click the orange button at the top right "Publish to X assets" to make public your changes.