wp_ajax_update_plugin

function wp_ajax_update_plugin() {
	check_ajax_referer( 'updates' );

	if ( empty( $_POST['plugin'] ) || empty( $_POST['slug'] ) ) {
		wp_send_json_error(
			array(
				'slug'         => '',
				'errorCode'    => 'no_plugin_specified',
				'errorMessage' => __( 'No plugin specified.' ),
			)
		);
	}

	$plugin = plugin_basename( sanitize_text_field( wp_unslash( $_POST['plugin'] ) ) );

	$status = array(
		'update'     => 'plugin',
		'slug'       => sanitize_key( wp_unslash( $_POST['slug'] ) ),
		'oldVersion' => '',
		'newVersion' => '',
	);

	if ( ! current_user_can( 'update_plugins' ) || 0 !== validate_file( $plugin ) ) {
		$status['errorMessage'] = __( 'Sorry, you are not allowed to update plugins for this site.' );
		wp_send_json_error( $status );
	}

	$plugin_data          = get_plugin_data( WP_PLUGIN_DIR . '/' . $plugin );
	$status['plugin']     = $plugin;
	$status['pluginName'] = $plugin_data['Name'];

	if ( $plugin_data['Version'] ) {
		/* translators: %s: Plugin version. */
		$status['oldVersion'] = sprintf( __( 'Version %s' ), $plugin_data['Version'] );
	}

	require_once ABSPATH . 'wp-admin/includes/class-wp-upgrader.php';

	wp_update_plugins();

	$skin     = new WP_Ajax_Upgrader_Skin();
	$upgrader = new Plugin_Upgrader( $skin );
	$result   = $upgrader->bulk_upgrade( array( $plugin ) );

	if ( defined( 'WP_DEBUG' ) && WP_DEBUG ) {
		$status['debug'] = $skin->get_upgrade_messages();
	}

	if ( is_wp_error( $skin->result ) ) {
		$status['errorCode']    = $skin->result->get_error_code();
		$status['errorMessage'] = $skin->result->get_error_message();
		wp_send_json_error( $status );
	} elseif ( $skin->get_errors()->has_errors() ) {
		$status['errorMessage'] = $skin->get_error_messages();
		wp_send_json_error( $status );
	} elseif ( is_array( $result ) && ! empty( $result[ $plugin ] ) ) {

		/*
		 * Plugin is already at the latest version.
		 *
		 * This may also be the return value if the `update_plugins` site transient is empty,
		 * e.g. when you update two plugins in quick succession before the transient repopulates.
		 *
		 * Preferably something can be done to ensure `update_plugins` isn't empty.
		 * For now, surface some sort of error here.
		 */
		if ( true === $result[ $plugin ] ) {
			$status['errorMessage'] = $upgrader->strings['up_to_date'];
			wp_send_json_error( $status );
		}

		$plugin_data = get_plugins( '/' . $result[ $plugin ]['destination_name'] );
		$plugin_data = reset( $plugin_data );

		if ( $plugin_data['Version'] ) {
			/* translators: %s: Plugin version. */
			$status['newVersion'] = sprintf( __( 'Version %s' ), $plugin_data['Version'] );
		}

		wp_send_json_success( $status );
	} elseif ( false === $result ) {
		global $wp_filesystem;

		$status['errorCode']    = 'unable_to_connect_to_filesystem';
		$status['errorMessage'] = __( 'Unable to connect to the filesystem. Please confirm your credentials.' );

		// Pass through the error from WP_Filesystem if one was raised.
		if ( $wp_filesystem instanceof WP_Filesystem_Base && is_wp_error( $wp_filesystem->errors ) && $wp_filesystem->errors->has_errors() ) {
			$status['errorMessage'] = esc_html( $wp_filesystem->errors->get_error_message() );
		}

		wp_send_json_error( $status );
	}

	// An unhandled error occurred.
	$status['errorMessage'] = __( 'Plugin update failed.' );
	wp_send_json_error( $status );
}