responsive tabs (tab to accordion on small screen width)
Short way : download Easy Responsive Tabs to Accordion Jquery plugin, add it to your theme and call the easyResponsiveTabs function for your HTML tabs markup.

Drupal Solution with Panels : use Panels responsive tabs to accordion, a sandbox project which provides a Responsive Tabs to Accordion Panel style. 


Install Panels module.
Install the sandbox project. This was a quick done project (no time to develop the admin options) so you may have to modify a few lines in responsive_tabs.inc to set up the paths to css or js files and/or HTML markup in responsive_tabstoaccordion_panels.js.
Go to  your region Panel style and choose the new style Responsive Tabs to Accordion