Check existing usernames on the Linux server before and after.

cat /etc/passwd | awk -F: ‘{print $1}’

Add a new username:

useradd -m <newadmin>

passwd <newadmin>

Enter password twice.

Add same username in XMC under Administration>Users and associate with NetSight Administration group. 

Logout of XMC and test login with new username.

https://extremeportal.force.com/ExtrArticleDetail?an=000077421&q=netsight%20user