I have registered at Youtube by using my gmail id. I would like to change my username name which is now “MY Full Name”. How can I change it?

As it uses your Google account, simply change the display name within your Google options.

As it uses your google account, simply change the display name within your Google options.

Thanks for your reply. Can I do it from Youtube account? Because I don't want to change my gmail name.

No, as they are both the same account.
This is ONLY your display name, nothing else.