Just loosen up the set screw on the left rear side of the reciever and you should be able to loosen the end cap and pull the plunger right out. Pull the trigger back at the same time. I wouldn't be surprised if that breech plug is stuck the way things look so you might want to get some good penetrating oil in their and get it soaking. If you don't have a breech plug tool you will have to get one or make one that will work.
I don't like the looks of that breech area too much, I hope the bore is better.