Hi again, Dave!
The instruction depends on the exact X-Cart 5 version you run.
If it's 5.2.7 or higher,
please follow this guide:
If your store is of an older version ( 5.2.6 or less)
please take steps below:
0) we always recommend that you do a backup first, whatever changes to the code you're trying to implement, whatever software you're modifying. But I believe this is obvious, so keep reading=)
1) you should change the value if the constant in file /classes/XLite.php:
const ADMIN_SELF = 'admin.php';
2) after that rename the file itself from admin.php to the one you need.
3)The reference to admin.php is also hardcoded in files below, update the link there too:
These 2 files may be in your store, and may be not - it depends on the set of modules installed. There may some other files I'm not aware of.
4) finally, rebuild cache. You will not be able to login to the admin back-end so far, to launch cache-rebuild via the web interface. Instead, you should remove the var/run directory and file var/.rebuildStarted, and after that login to the store using a new URL. At this moment the store will not be able to find cache, so it will rebuild it. Cache rebuild may take a couple of minutes, BTW.
PS the X-Cart community forum may be a better place to ask your further questions. There are many 3rd party developers and X-Cart own engineers, who constantly check the new threads and answer questions. I do monitor all X-Cart mentions and find questions with Google Alert, but I'm afraid I may miss a question one day, so it's better to rely on a more X-Cart specific place to ask;) But anyway, I'll try to be here to help, too.