Alter $WP_Query in WordPress

When showing list of content (i.e. Posts, Custom Post Type) on Category Page, the main $WP_Query renders posts of default Posts type. To show content from different post type we've to alter the main $WP_Query.

There are few different ways using which we can alter the $WP_Query. But here is one of the way to alter $WP_Query without removing existing conditions in Query.