How to re-edit /system file to reboot phone? How does one re-set or edit the /system/build.prop, when changes kept it from booting up again? Rooted Optimus.
Try rebooting into recovery and connecting to the phone via ADB:

adb devices

It should give a message like this:

List of devices attached  
01234565ABCDEFG        recovery


You can then copy the file off the phone:

adb pull /system/build.prop c:\build.prop

You can now edit it in any text editor (recommend Notepad++ because it will preserve the Unix style line endings,) and then copy it back to the phone:

adb push c:\build.prop /system/build.prop

If the issue was indeed within that file, the phone should boot now.