A YaST Update is a Update Medium that contains a directory with the name of y2update. The directory contains a file hierarchy identical to that under /usr/share/YaST2. It can be used to replace config files or YaST2 components from the CD with new versions.
For a YaST Update only it is sufficient to have a directory y2update in the base directory ( see Section 2.1) of the Update Medium. Everything present in this directory before YaST starts will be used by the installer.