Taking the Barrell in and out is easy when you've done a few like everything
To get it out, Key In, turn to pos 1, press slightly and turn Retaining Ring, now, still in pos 1 pull the Key and Barrell comes out with it !!
Now the Key will pull out of Barrell for working on it.
To Replace, put Retainer Ring over Key, insert Key into Barrell, hold it above the Lock Aperture, at pos 1 position, push Barrel etc in and locate, whilst also locating Retaining Ring, and pressing on Retaining Ring and turning to retain it all in place.
Turn to off position and remove key !! Now re insert Key and test it is all working OK
I can't remember which way to turn the Retaining Ring, but unless it's been parked in the sea and all corroded they come off really easy, so try it both ways.
HTH, Cheers Dave