Where can I change email address for administrator notifications?

Default email address is used from general WordPress settings: “Settings” > “General” > “E-mail Address“.

If you need to change this email you need go to the plugin settings: “Contact Form” > “Settings” > “AutoResponder” tab > “Admin AutorResponder Notifications” section > “Send TO Emal” field.

More information you can find in the plugin documentation in following sections: AutoResponder Tab and Admin AutoResponder Notifications.