It's held in with a steel plug. You just pry it out then push the plug back in after you change the spring. One tip, if the spring is broken in many pieces and doesn't come out on it's own, slide a wood screw in there and the threads will catch the spring and you can draw it out.
Just put the new spring in the roller housing and press the plug back in, then put it all back together.
2002 BMW M3 - Stock