Next Go to your backend to access files using FTP, themes > mist > includes > find theme-functions.php. Rename this file as theme-functions-old.php. Place the new downloaded file(theme-functions.php) here.
After that Go to admin backend > login to admin > Go to Theme Options > General > Mailchimp API > Remove the key show in the field.
Now again go to file backend using FTP, themes > [ installed theme ] > includes > find theme-functions.php. Delete this file. Then Rename your theme-funtions-old.php as theme-functions.php.
Now check it in front end and keep the Mailchimp API field empty or enter your own Maichimp Key.