*** UPDATE *** As of Divi 3.0.1 you can now edit the Divi footer credits using the Theme Customizer.
To edit the Designed by Elegant Themes | Powered by WordPress in your footer, you will need to edit the footer.php file. The best way to do this is in your child theme so that it doesn’t get wiped when you update your master Divi theme.
In your child theme copy over the footer.php file and look for;
then just add your text in so it looks like;
<p id=“footer-info”>ADD YOUR TEXT INTO HERE</p>