There are two ways how to update GDayNews theme. You can use Envato WordPress Toolkit plugin, but if you don’t want to or cannot use that plugin so you can use your FTP account.
This plugin is installed with GDayNews theme by default. If you don’t have installed it from some reason, simply download it from here: https://github.com/envato/envato-wordpress-toolkit and install it by standard way.
An access to your FTP account you should get from your hosting service.
This video is for FlyingNews theme, but way how to update theme is the same also on GDayNews themes.