How can I add new link to my nav bar in the header when customizing the help center?

Hello I want to put an extra link in my navigation bar (nav.user-nav) I have attached an image - this link we want to call "Quick User Guide" and have a link to this. I have looked in the forums but have found nothing specific. I can make the extra link/name by copying and pasting one of the others in the Header file etc but can't seem to add the url etc... Do I need to access another html file etc?

Sorry if this is doubling up but I couldn't find what I was looking for so thought a post may help me.

Many thanks Lisa