How to change Helo Address?

You can change it in postfix configuration file /etc/postfix/main.cf.

The helo line is controlled by the smtpd_banner parameter. It probably references the myhostname option. Put your server name there instead of localhost:

myhostname = server.example.com
smtpd_banner = $myhostname

and restart postfix with sudo postfix reload.