Editing the sidebar

use a link to a page on your wiki, but replace your wiki page's title (e.g. Main_Page) in the URL/Address bar with MediaWiki:Sidebar and load that page - you should now see the current content of the sidebar. (You may need to use the syntax //yourdomain/yourwiki/index.php?title=MediaWiki:Sidebar&action=edit if you use long URLs.)