Edit the file 
sudo nano /etc/webmin/miniserv.users 
and add a line for your new user like
anubhav:newpassword:0
Edit file 
sudo nano /etc/webmin/webmin.acl 
and copy the first line for the root user and paste it in same file , rename root to the new user and save the file, so the file should be like
root:access privileges
anubhav:access privileges
Reset the new user password
sudo /usr/share/webmin/changepass.pl /etc/webmin admin newpassword
Restart webmin from 
sudo service webmin restart
Navigate to your webmin page and login user new username and password.

root@servertwo:~# sudo su
root@servertwo:~# nano /etc/webmin/miniserv.users
root@servertwo:~# nano /etc/webmin/webmin.acl
root@servertwo:~# service webmin restart