The Extra Theme does not have an option in the Theme Customizer to change the bottom copyright text. You can follow the easy steps below to change the footer links in Extra.
You need to add a copy of the footer.php file in your child theme, here is a detailed tutorial from our blog to learn how to make a child theme https://www.elegantthemes.com/blog/divi-resources/divi-child-theme
You can also download a fresh Extra Child theme from here: https://www.dropbox.com/s/io243kpu9ksj6wh/ExtraChild.zip?dl=0
Once you have a copy of your Extra theme's footer.php file in your child theme, please look for this part of the code:
<p id="footer-info"><?php printf( et_get_safe_localization( __( 'Designed by %1$s | Powered by %2$s', 'extra' ) ), '<a href="http://www.elegantthemes.com" title="Premium WordPress Themes">Elegant Themes</a>', '<a href="http://www.wordpress.org">WordPress</a>' ); ?></p>
You can replace it with
<p id="footer-info">Your Own Footer Text Here!</p>
That's it! save the file and you are done!
Note: If you modify any template files like the footer.php without a Child Theme, directly in Parent theme files, then you will lose those changes upon updating.