how do I get the plactice knob off the faucet?

Hi Bina, Thank you for your question. To remove the handle, pop off the top cap, then remove the Phillips head screw. Please make sure that the water supplies are turned off before disassembling the faucet. To access the seats and springs, unscrew the bonnet and lift out the ball (1-handle) or stem unit (2-handle). Best regards, Jon