How to change your name in Japan

Hello everyone!
I've got a (weird) question.
My friend (she's (half) Japanese but has the German citizenship), has got a German name but she'd like to change her name in Japanese. (ex. let's say her German name is "Julia Müller" and she wants to change her name to "Tachibana Hanako")
She wants to live in Japan again (we plan to go together) but she'd like to have a normal Japanese name since she will live in Japan.
So... How do you change your full name in Japan or in Germany? (In Germany, it's quite difficult to change one's name)
What is required? How about the cost?

We keep looking for answers!

If anyone knows, please don't hesitate to tell me!

Inquire at the municipality where her koseki is registered.

Unless your friend can get/have Japanese citizenship, dont think she can get her name change in Japan. If she used to stay in Japan and registered in koseki, she can dig up that koseki and see what her registered name was.
So no Japanese citizenship, no can do in Japan.

Only in Germany then.

But she can apply for alias if she can proof that Japanese name is used regularly.