I assume you are referring to the system login password.
If you want to change your password at the command line, after you log in, open a terminal and run the command "passwd." It will prompt you to enter your old password, then prompt you to enter your new password twice. Then log out and log back in to test the password.
If that does not produce a password that sticks, please provide more precise details about the wording of error messages you are getting and the precise circumstances under which you are getting them.