1. Most of the templates have a menu that can be edited by editing the HTM template. So, open the template code.
Dashboard → Themes → Click on the three-dot menu → Edit HTML in the new Blogger
Dashboard → Templates → Edit HTML in legacy Blogger
In most cases, the code to look for looks something like this:
<li class='current'><a expr:href='data:blog.homepageUrl'>Home</a></li>
<li><a expr:href='data:blog.homepageUrl + "feeds/posts/default"'>Posts RSS</a></li>
<li><a expr:href='data:blog.homepageUrl + "feeds/comments/default"'>Comments RSS</a></li>
<li><a href='#'>Edit</a></li>
There you can replace expr:href='…', with your link: href='http://mylink…'. Please check your template instructions for more information.
2. Some templates include a link list gadget or a page gadget that lets you edit your menu right from the Dashboard.
For the link list gadget: Dashboard → Layout.
For the spreadsheet list gadget: Dashboard → Pages → New Page.