I've written email to admin@zymic.com, but no one reply to me.
Hope I can get help here.

Dear Zymic administrator,

My username is "madoka".
I've registered a web hosting account "kinki-dreamland", but I want to change the name into "my_wish".
Could you please update it for me?
Thank you very much!

Madoka

Can't change the name of accounts, just delete all the files on that one and leave it be, then create a new one.