1) Install the same ispconfig version on the second server
2) move all lines for the web* and client* users and groups from /etc/passwd and /etc/shadow and /etc/group and /etc/gshadow to the same files on the new server
. Copy just these lines and not the whole files!
3) Copy the /var/www and /var/vmail directories to the new server
. Make sure that you preserve permissions by e.g. using these tar commands:
tar pcfz www.tar.gz
tar pcfz vmail.tar.gz /var/vmail
4) Move the ispconfig mysql database to the new server
and also move all client databases to the new server
To your second question: To update ispconfig, run the commands:
as root user on the shell.