I am trying to change the domain of my website

I need a help to change the domain of my website.
I am currently using two domains and one is as the sub-domain.
But I would like to use the sub-domain as main. (current website will be shown as katsupiano.com)
Also, I would like to duplicate the website and use my current main domain (katsuakitakizawa.com) as Japanese website.

I tried to do by myself, but I couldn't figure out how to do it.

Thank you.