If you are savvy with CSS, you can target the slider widget using its unique ID using Inspect Element in your browser, then reset the position and margins.
If this is over your head, you can get a full-width slider for use later down the page by installing a slider plugin. Please note that plugins will not work with the theme’s slider post types, and will either install their own post type, or will provide options for displaying full-image slides instead.
What We Can Recommend
The Meta Slider plugin works well for this purpose and is actively developed and compatible with Express. For best results, set it to use the Flexslider at 1680 or 1920 width to account for large screens (it will scale) then add the following to your Custom CSS box in Theme Options:
.widget_metaslider_widget{padding-left: 0px!important; padding-right: 0px !important;}