i launch WCS5 on Amazon EC2, i access server from CLI with key pair (name is "flashphoner.pem") by aws supply. I want to change password of demo account but i can not. An error occurred when i entered "passwd demo". Unknown user name 'demo'
You're trying to change a user in the AWS Linux system, but there's no user demo on this level. You should enter to WCS CLI using admin password (which is usually equal to instance Id)
ssh admin@localhost -p 2001
Please note that for security reasons CLI port is available on localhost only, so you shoul SSH to the instance first (as you've already done according to screenshot)
Then, you can use WCS CLI command passwd to change demo user password