add navigation tab at top

Hi and thanks for the help up front. I used to use WP a good bit, but have been on other projects. I knew the answer to this at one time and have forgotten it. Thanks up front for the help. The client wants to add another tab to the navigation, We have the page ready and just need to get it on the main navigation. I know asking is lame but I am behind on two projects so I thought I would ask here.
Thanks again and peace…