Busybox 1.20.0 update !
If you are happy with 1.19.3 don't use this !
This update package roots your phone + installs Busybox 1.20.0
No need to install busybox separately later... This is the latest version & full featured.
OK, So what you need to do is :
1. unroot using unroot.zip from OP.
2. Then flash update.zip from here as described in OP.
Make me know if there's any problem !
OP conditions apply !