Hi Ed,

The answer depends on your web host details.

Generally it should be done in next steps:
1) at the "old" domain (sub.domain.com)
- login as user with uid=1 (first drupal user created at your site) and turn your site (sub.domain.com) off-line for maintenance
- create backup of your database (for sub.domain.com)
- create backup of folder 'sites'
- for drupal 5.x: create backup of folder 'files'

2) transfer backup files from sub.domain.com to domain.com

3) at the "new" domain (domain.com)
- recreate database from your backup
- extract drupal-6.x.tar.gz into right place
- recreate content of folder 'sites' from your
backup (for drupal 5.x, folder 'files', too)
- access your "new" home (http://domain.com) page via browser - probably your site is "off-line for maintenance" and you should turn it on - if you have problems, you can find help at http://drupal.org/node/56995

4) test your new site

5)
if ($everything == "OK") {
enjoy();
} else {
locate_and_fix_the_problem();
}


Notice 1:
if you are hosting both sub-domain and domain on the same host, maybe it's easier to re-configure (somehow) your web host or to copy whole content of your drupal folder to new location.

Notice 2:
if you have "hard-coded" some internal links (link to something like http://sub.domain.com/node/11 ), you should edit those links to new domain (convert to http://domain.com/node/11 )

Important:
Whatever you try, first of all, create backup (both, database and files)!

Additional resources:
* Back up your Drupal site
* Backup Process with phpMyAdmin
* Moving Entire Drupal Site with Databases

Hope that my answer will help you.

Best regards,
Alan Fork