Upgrading to a later Maintenance Release
The third version number is known as a maintenance release and should only ever require a simple update to perform. Maintenance releases never contain any schema changes, and do not require any sort of "updater".
For example, upgrading from 6.0.0 to 6.0.15.
Steps to Upgrade
- download the maintenance release; this will only be the ./core directory (and all its underlying file system)
- Perform any backups (optional: maintenance releases are normally very stable and don't make data or schema changes, still it never hurst to be cautious)
- replace your current ./core directory with the newer maintenance ./core directory structure
- restart the application (eg. ?updateapp=yourappkey, or from the editors tray, or from the webtop admin tools)
- that is all!
Make sure you do a complete replace of the ./core directory. Depending on your operating system, just copying over the top of the directory structure will append and replace. This would mean you never delete old and dead files in core, which may have serious repercussions ie. your app breaks. Just be sure to replace the directory structure and all will be well 
.