Pull the plug
unload, unscrew the barrel nut (the one that holds the barrel on), use a screwdriver to gently pry out the steel insert (from inside the magazine tube) Be careful as the spring will jump out and dot your eye. Plug will come out with the spring.
Replace the spring, compress and reinstall the insert (you can use a screwdriver handle to gently tap it into position) replace the barrel nut, good to go.