How do I update my logo in the header?
To update your logo in the header, go to any page that uses the theme and click on the header global group. You'll be prompted to open it with a message that says "Any changes made to this content will apply to all instances of it in your account. You'll be able to preview your changes on all assets before publishing.", click on "Open in global content editor".
Once you're in the editor, click on the logo to update it. A logo image module will open in the sidebar that will allow you to replace it.
If you've deleted the logo in the past and can't find it, click on the "Contents" tab in the sidebar which should give you a list of all of the modules in the header global group. Click on the "Logo" module to update it.