How to change the default username to a different user?

How to change the default username to a different user?

Hi,
 
Currently we have 3 users in our account. We would like to keep all 3, but change the current default user to another user so that the other one is the default user.
 
I am using the free version of ZOHO.
 
Can somebody please help?
 
Kind Regards,
 
Vanessa