You have to grab the knob with one hand, and pull up while rocking it side to side. I just had mine off on my BX23S to install the replacement loader valve knob. The plastic is retained by 2 short bolts on the BX23S, one in the middle at the top, and one on the right side about 3" down from the top. Both screws require that you remove the rear wheel and tire to access them. At the bottom of the plastic panel, are 2 tabs that fit into the steel fender, and you lift the plastic from the top towards the front of the tractor to disengage these tabs. Also makes it easier if you remove the rubber boot from the shaft once you have the knobs off.
Looking at the picture you posted, there are a bolts that can be accessed from the top making it a lot easier than the BX23S. I went into the detail for the BX23S because I know that this question is going to be researched at some time in the future, and it might be helpful to both you and others that come to this thread later on.
Looking at the picture you posted, there are a bolts that can be accessed from the top making it a lot easier than the BX23S. I went into the detail for the BX23S because I know that this question is going to be researched at some time in the future, and it might be helpful to both you and others that come to this thread later on.