How to change account user

How to change account user

Hi,

I have 5 users, but can't create user after updrage to Standard.

How to change account to standard from info@pro-audio.vn to an.nl@pro-audio.vn

info@pro-audio.vn		0 GB	30 GB	0.16 GB	0.00 GB	0.16 GB
an.nl@pro-audio.vn	No Mailbox
my.nl@pro-audio.vn	No Mailbox
duc.nl@pro-audio.vn	No Mailbox
sales@pro-audio.vn