I have registered few domains from https://www.tierra.net/ .But along with that I have also registered one domain for my friend.And I am its legal owner.But now he want his ownership. So what I can do ?
It depends on what kind of tld it is - and how the registrar handles it. Is the domain locked or unlocked. Do they allow free push... Write them a support mail, and tell them you need to push your domain to a new owner ... and what approach that would be best. Presumably your friend could simply open an account, and you could push it to him.
I think all registrars allow free push so its easy. Like pj1s said tell your friend to open and account there and push the domain to his user ID.
If you have a "problem" with your friend wanting ownership of the domain, talk to him about it. Otherwise, you can transfer the domain name to him as other people here have posted.