1) Carefully insert a fine pointed instrument such as a compass point or probe from a multimeter (as shown below) into the release hole of the bakelite mouthpiece. Be very careful not to damage the bakelite.
2) Applying gentle pressure to the release spring inside the mouthpiece rotate the bakelite mouthpiece anticlockwise by 1/2 an inch (as shown below).
3) At this point the bakelite mouthpiece should just lift clear of the main telephone handset.
4) The carbon granule microphone is now exposed.
5) Replacement of the mouthpiece is simple, just put the bakelite mouthpiece on and rotate it 1/2 clockwise (if adjusted correctly it will lock onto the telephone handset).