That round disc you see is a small aluminum cup which is lightly pressed, face down, into the cast aluminum housing. It's held in there by friction.
To take apart this type of tuner you first remove the small screw in the end of the knob. Pull the knob and the plastic friction washers off of the end of the key shaft. Push the key shaft out the right side of the housing. The worm and all will go with it.
Then, set the housing with the post up on something that has a hole in it about the size of the little disc. Lightly tap or press the post shaft straight down, and it will push the little disc out the back of the housing. It's not in there very tight. Then you can pull the whole main gear and post out the back of the housing.
Reassembling is the reverse. Use a block of plastic or wood to tap the disc back into the housing. Try not to scratch it all up; it's soft polished aluminum.
Once you have it apart, what do you plan to do? You can clean it, re-grease it and put it back together. I don't think you can buy any replacement parts.