Solaris

Ok, I was able to revert it...
I logged with another user and used su
it requested root passwd and allowed me to modify the passwd file again...

to be honest, I am just trying to learn about it...

what should be the modification in passwd file If I want to change the shell used for root?

Also, what is the differences between them?

Thank you!!

I understand that you are learning, so here are a couple of lessons to get you started.

Root is the super-user in Unix, when working as root there are very few safety checks performed.
Don't use root unless you have to, you are likely to break your machine if you do and are inexperienced.
Don't log in directly as root, use 'su' from a normal user.
Don't change the root shell, it can be done easily but there is no need to do so. If you have to work as root and want a different shell then just type the name of the shell you want to use (eg bash)
Do use and customize a normal user in whatever way you want, including the shell you want to use, the usermod command below will work for you.