这样的sed命令如何写？

fbt045

各位高手，我有如下文件：
[test@ ~]$ cat data3.txt
On Tuesday, the Linux System
Administrator's group meeting will be held.
All System Administrators should attend.
Thank you for your attendance System
Administrator

想替换掉所有System Administrator为Desktop User，但以下命令却没有替换最后一个符合的字符串，请问这是什么问题，需要怎么改命令？多谢哈
[test@ ~]$ sed 'N; s/System.Administrator/Desktop User/g' data3.txt
On Tuesday, the Linux Desktop User's group meeting will be held.
All Desktop Users should attend.
Thank you for your attendance System
Administrator