Undo the top allen bolt. The shifter will drop off the brake perch. Then undo the screw holding the plastic cover on the bottom. This will expose the innards. You should be able to see the pawl and manipulate it with a little screwdriver. It should flick freely. I have had to pull the circlip off and remove the pawl to grease the shaft it pivots on. Fiddly, but possible.