All You need to do is
a) login as root and use konsole
passwd usernamereplace
username with actual login of the user You want to change password for and press enter. Now type in new password twice confirming each time by pressing enter and You're all set.
You can use su -c "passwd username" command instead of logging in as root...

b) Use
Kmenu > More applications > Configuration > Configure Your computerAnd on the
System tab click on
Manage users on the system entry.
Right click on the user You want to change password for and choose
Edit from the menu.
I am sure You can handle the rest of the procedure

.
Regards.
Andy