You don't use text editor to copy to SD CARD here are the steps you may take.
Open up terminal emulator or adb shell whichever you perfer
$su
#cd /system/
#cp /system/build.prop /sdcard/build.prop
Then plug your phone into your computer or use ASTRO file manager and open the build.prop (on your computer use notepad on ASTRO use text editor) -- save your changes.
$su
#dd if=/sdcard/build.prop of=/system/build.prop
#sync
#reboot
Like someone mentioned earlier, its way easier to use root explorer to do it.
Open up terminal emulator or adb shell whichever you perfer
$su
#cd /system/
#cp /system/build.prop /sdcard/build.prop
Then plug your phone into your computer or use ASTRO file manager and open the build.prop (on your computer use notepad on ASTRO use text editor) -- save your changes.
$su
#dd if=/sdcard/build.prop of=/system/build.prop
#sync
#reboot
Like someone mentioned earlier, its way easier to use root explorer to do it.