I found it on line 50 in template-functions-general.php in /wp-includes/. You can just change those words, keeping what you insert between the single quotes and inside the parentheses.
Keep in mind, this is known as “hacking core code”. You’ll have to redo this tweak every time you upgrade. A lot of folks make evil-eye wardsigns at people who hack core files. I’m not one of them…. I hack core when I need to.
What you REALLY need to do is leave a couple of blank lines before and after, and include a comment about what you did, so you’ll be able to find it; also, I usually keep a text file with a list of files I’ve hacked, the line number, what was in it originally, and what I replace the original with. It’s safest.
So here’s how you could do this – replace line 50 in the template-functions-general.php file with the following (making the words say what you want them to):
/* Mod on 12/23/05 to change Site Admin to Bhoney's Admin - begin */
$link = $before . '<a href="' . get_settings('siteurl') . '/wp-admin/">' . __('Bhoney's Admin') . '</a>' . $after;
/* Mod to change Site Admin to Bhoney's Admin - end */
