We're celebrating Europe this week and we'd like to support all of our European Union customers by covering the VAT rate. Discount is valid for all new purchases and manual renewals and applies automatically during checkout.
I don’t see a way to change the used tags using the widget_title filter (it can only be used to filter the title text, not the wrapping tags).
The second bit of information suggests replacing the theme’s sidebars registration code entirely. You can find that code in Mantra’s mantra_widgets_init() function in includes/theme-setup.php.