pmod <name> <old_password> <new_password>

[root@sylixos:/root]# help pmod           
modify a user password for this machine.
eg. pmod root root newpass
pmod name old_password new_password
[root@sylixos:/root]# 

[root@sylixos:/root]# pmod liang liang 1234
[root@sylixos:/root]#