Should be an easy job. Have a look on youtube for ak breakdown videos.
Do you mean the whole inner barrel, the thing that should be attatched to the hop unit, that the bb's roll down? Never had that happen to be honest as it should be a very tight fit to the hop unit. I would imagine you are missing the small coller for the hop unit, or you have had a massive hop rubber failure (ie its not there!) to allow the inner barrel to fall out of the hop.