How to update theme?
Go to Downloads section of your ThemeForest account and download BigBang Wordpress theme.
I prefer FTP overwriting all theme files. To do that, unpack the file and unpack the file bigbangwp.zip. You’ll see bigbangwp folder. Upload this folder to your server to wp-content/themes/ and overwrite all.
Another way is to delete current version from the WP Dashboard and upload bigbangwp.zip file.
All settings are in database so there’s nothing to worry.
If you’ve changed some core files, you should upload only new files. These files are listed in style.css in theme root folder (bigbangwp)