[Danke]

You'll have to detach the hopup from the inner barrel. That's a very tight fit and you'd never get it in the right spot trying to work it through the outer.

Wind the hopup off so the arm sits as high as it goes and pop the C-clip from the inner. When you pull the inner and rubber out the only other thing that may come free is the nub so hold it upside down to reduce the chance.

Getting the new rubber and barrel back in will take patience and gentle steady pressure. When you have things apart you'll see the notch in the barrel the nub presses on, that has to match with the nub and arm when it's all back together.

When it's all back in snap the C-clip back onto the barrel and look through from the feed end and double check that when you adjust the hop you can see the nub pushing the rubber down.