You have to replace the cable. It may be easier to pull the carb off the mount and you can probably leave the cables attached to access the choke cable on the carb end. Removing the fuel line will help too.
It's an easy process once you visualize the hookup on the carb and then disconnect it from the carb. You thread the new cable through the bracket on the bike after removing the old one, install the locknut on the cable and hook up the carb end. Just make sure to only tighten the nut finger tight and only a tad more if the choke knob doesn't stay out when you pull it out on it's own.