You might want to start here,
You should remove packages for Slackware with the removepkg command, as root, of course.
To change a icon... I'm not in KDE, but IIRC, right click on the start button, the K in the lower left hand corner. Pick edit programs (something like that).
Find the one you want and edit its properties. Again, IIRC, you right click on
the box for the icon and then browse around until you find the icon you want. The system will first show you some default icons, but if what you want is not among them, then you can browse to where the icon you want is stored on the hard drive.