After removing the screw you can gently pry or pull the old knob off. Now would be a good time to clean out the crevices on the radio where the old knob sat.
You will now want to get your newly printed knob. Use the .stl file at the bottom of the page to either print your own or have it printed and shipped to you.
I printed this one in ABS but it would probably be even better in PLA. I printed this one with the top on the print bed. (upside down)