The default sender is set in Preferences > General Settings > Email tab.
You can set the default sender display name and sender display email here. As a side note, you will want to set up your information for your email account in this area. If you do not, then the email notifications generated by the system will continue to be sent from our default email account. You will never receive any replies that your customers may make to the system generated emails. You can override the default sender display name and sender display email on a site-by-site basis in Sites > [site name] > Settings > Email Notifications.