Okay. I just asked, because the default template has no tabs ... ;-)
So, I suppose, what you mean is the index *button*!?
You can edit your template's
main.php[/m] and add a button on your own. You can use the function [m]html_btn() which DokuWiki provides like this:
print html_btn('description','pageID','',array());
For less trouble when upgrading it is always advised not to change the default template but to make one of your own. This can simply be done by copying the default template's folder and renaming it.