You don't mention the year of your car--there are different types--the earliest with the "soft" knobs have a small phillips head screw through the center--you have to pop out the little disc with the wiper icon--the bottom of the knob has two little extensions that fit into slots in the switch shaft--be sure to hold the knob part still while loosening the screw or the knob will twist those pieces off faster than you can say "oh darn it". They are necessary to operate the washer part of the switch.
The later switches with the "hard" knobs ('71?-'72?) simply unscrew from the shaft with a built in stud.
After the knob is off the slotted retaining collar unscrews from the switch.--Dave