Boot into recovery.
Go to advanced, and click ‘mount’ and select system.
Plug your phone into your computer and open your terminal/cmd run the following:
adb pull /system/build.prop
This will pull the build.prop onto your computer in the location where your terminal is open. Edit that in your favorite text editor on your computer and then add the following line:
Save, and then run:
adb push build.prop /system/
This puts it back on your phone, but the permissions might be wonky, so run:
adb shell chmod 644 /system/build.prop