Notify the system that preferences were changed
Your question needs some editing and clarification.

What I think you're asking is, how to force OS X to re-load data after editing a plist file.  There are two methods of doing this.

OS X uses a caching daemon for preferences, cfprefsd.  You can simply save your plist file, then do  sudo killall cfprefds.  It will restart automatically, and should load your changes.

The recommended way of forcing changes is with the defaults command, which informs the daemon of the changes. For example


# Reveal IP address, hostname, OS version, etc. when clicking the clock
# in the login window
sudo defaults write /Library/Preferences/com.apple.loginwindow AdminHostInfo HostName