Well, for starters… I wouldn’t touch that theme editor: it doesn’t have an undo button!
I always edit my template files on my computer in a plain text editor (Notepad would do it).
1. You download the template files from your theme:
example.com/wp-content/themes/default through a FTP client/sotware.
(the same one you used to upload the WP files).
2. Open the index.php file and find this in line 11: <!-- by <?php the_author() ?> -->
Make it to llok like this: by <?php the_author() ?>
3. Upload the modified file.
Note. It is always advisable to make a backup copy of all the files you modify – just in case something goes wrong, you can revert to the unchanged version.