Check this reference
http://www.wpbeginner.com/wp-themes/how-to-add-custom-dashboard-widgets-in-wordpress/
for adding dashboard widget
for removing dashboard widget
add_action('wp_dashboard_setup', 'example_remove_dashboard_widgets' );
function example_remove_dashboard_widgets() {
remove_meta_box( 'dashboard_quick_press', 'dashboard', 'side' );
remove_meta_box( 'dashboard_recent_drafts', 'dashboard', 'side' );
remove_meta_box( 'dashboard_primary', 'dashboard', 'side' );
remove_meta_box( 'dashboard_secondary', 'dashboard', 'side' );
remove_meta_box( 'dashboard_browser_nag', 'dashboard', 'normal' );
remove_meta_box( 'dashboard_right_now', 'dashboard', 'normal' );
remove_meta_box( 'dashboard_recent_comments', 'dashboard', 'normal' );
remove_meta_box( 'dashboard_incoming_links', 'dashboard', 'normal' );
remove_meta_box( 'dashboard_plugins', 'dashboard', 'normal' );
wp_add_dashboard_widget('custom_help_widget', 'Theme Support', 'custom_dashboard_help');
}
function custom_dashboard_help() {
echo 'Your custom banner code';
}