Change super administrator mail

Change super administrator mail

I've made a mistake and I set the administrator email to administrator@mydomain.co.il and i want it to be office@mydomain.co.il
how may i change it? I have 5 users limit, I can't use that email unless I'll change it to office.
should I delete all mydomain account or there is a way to just change the super administrator email?

Thanks,
Yotam Noy Levy