this is not a simple change to make.
you will need to access the power CMS database directly to edit the pcms2_contents table and change the page names in the ContentGroup column, or delete the records for pages that have been deleted, the problem with doing this though, is that the code on each page where you have changed the name in the ContentGroup column will need to be edited as well.
for example, on the about us page, the code to output the main content area will look like this:
<?php echo WA_getCMSContent($database_PowerCMSConnection, $PowerCMSConnection, 'About Us' , 'content'); ?>
If you change the name in the ContentGroup column of the database from "About US" to "All About Our Company", that code will need to change as well:
<?php echo WA_getCMSContent($database_PowerCMSConnection, $PowerCMSConnection, 'All About Our Company' , 'content'); ?>