How do I make changes to the header and not just a single page?
I want to make changes to the header and not just one page. Can I please get some input?
Presuming you have the same header on all pages, if you target your experiment to "all pages", then it will modify the header on all pages!
Perhaps this information on URL Targeting will help you:
You can choose sub-string match for URL targeting and put your website name on to it. This will alow the experiment to run on all pages of your site. Then when you are on the editor mode, go to editor setttings and display any page from your site by putting an URL (you can put home page too). Then choose the header section and make the changes. It will be implemented on all pages of the site.
I have attached screenshots for your reference.