Some screw on, probably just older ones.
Most of the newer ones are just forced onto a splined end (straight grooves). The splines just tear, or "eat", into the molded plastic of the knob when it is pressed on. Once you take it off, it won't stay on again if it is this type. Look at the new one and see how it attached and maybe check the diameter. Then, if you are up to it, pull the original one off... or unscrew it...
