accessing monthly archive from dashboard

the html for any page is in the template

Often for archives, that is handled by archive.php, but you can make sure that is correct following the template heirarchy
http://codex.wordpress.org/Template_Hierarchy

That is the basic html, access from appearance->editor from your admin dashboard

Each template has the code, but you may have to dig further. The template will usually call other html, using get_header, get_footer, get_sidebar, etc

This calls in other templates, like header.php, footer.php, etc

And a newer function is get_template_part which can call in other templates, like loop.php, etc

I explain how that works here
http://voodoopress.com/2011/02/get_template_part-in-my-wordpress-theme/