All You need to do is
a) login as root and use konsole
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 computer
And on the System
tab click on Manage users on the system
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