How do I edit the "submitted by" author info in node posting for this theme?

Edit node.tpl.php and change the following:

<?php print $submitted ?>

to

<?php print str_replace("Submitted by", "whatever you want", $submitted) ?>

Or more technically, if you want support for more languages:

<?php $submitted123 = t('Submitted by');
print str_replace($submitted123, "whatever you want", $submitted) ?>

Make sure you get the case and spelling of "Submitted by" correct, in whichever form you use.

Let me know if this doesn't work. I'm away from my main computer at the moment so I haven't had a chance to test it.