Open up your favorite FTP editor, and connect to your site. Navigate to your theme folder.

Open up the functions.php file, and look for the function name the_copyright_text. Edit the string being assigned to the variable $footer_text.

You can also edit your theme files through the WordPress dashboard. Be careful, though, because if you make a coding mistake, your website will turn white, and you will not be able to log in without connecting to your site via FTP and fixing the code bug.