If I understand it correctly, you're pointing to your default account which is the system account. Why do you want to change that?
It's just system account.
You can still use your other account as the main account you mail from and also automatically forward messages to the system account to your main (not default) account.
As far as I know (if I'm not mistaken) you can't change this default.
I have the same issue, could you please help me with that...I wanted to rename one of my Email account name, but I am unable to do it. Please share me some info on it.
Please create a new thread for your question as it's something else. And if you're a customer (did not buy Directadmin but only use directadmin) you have to contact your hosting provider for support.