Source

File: wp-admin/includes/bookmark.php

function wp_update_link( $linkdata ) {
	$link_id = (int) $linkdata['link_id'];

	$link = get_bookmark( $link_id, ARRAY_A );

	// Escape data pulled from DB.
	$link = wp_slash( $link );

	// Passed link category list overwrites existing category list if not empty.
	if ( isset( $linkdata['link_category'] ) && is_array( $linkdata['link_category'] )
		&& count( $linkdata['link_category'] ) > 0
	) {
		$link_cats = $linkdata['link_category'];
	} else {
		$link_cats = $link['link_category'];
	}

	// Merge old and new fields with new fields overwriting old ones.
	$linkdata                  = array_merge( $link, $linkdata );
	$linkdata['link_category'] = $link_cats;

	return wp_insert_link( $linkdata );
}