I have tried to edit the “contact us” button external link in the header of each of my website pages and had no luck. Spoke with multiple representatives and was still unable to figure it out.
Since you can’t edit the contact us button via page editor, you can try to modify it via design-manager/design-tools. Look for the header.html which should be located in THEMENAME/templates/globals (path might differ) and you should see a banner like this
this should take you to the global settings of the header where you can apply your changes.