SEE: the comment by OGC Member. He explains how to modify the code in the footer.php file located in the Themes – Twenty Thirteen directory. Only the second line of his example code needs to be inserted and customized. Remember to backup the original footer.php file before you make your edits. I recommend that you install and use the app ‘Notepad++’ as your php files editor. Windows notepad is rather inadequate.
Example: You can add your owe copyright notice to replace the phrase Proudly Powered by WordPress.