Zen Cart 1.5.8 and above:

Open the includes/languages/english/lang.index.php file and find the following code:

'HEADING_TITLE' => 'Congratulations! You have successfully installed your Zen Cart® E-Commerce Solution.', Replace the text starting “Congratulations” with your own text that is welcoming and describes your business.
Make sure that the single quote marks at the beginning and end are not left out. If your text includes single-quotes (apostrophes) be sure to add a \ before them to “escape” them for proper PHP syntax.
NOTE: There is a similar line that starts with 'HEADING_TITLE_NESTED', so be sure to replace both lines.
I followed instructions and replaced text in both lines but text doesn't change. it is not a cache issue, tested in two browsers (Firefox and Chromium, refreshed and SOL).
This must be coming from a different file, which one?