to get the old barrel out you need to remove all the cowling asuming the scum havent allready.
you need to put your key in the barrel and turn it to the first position.
if you look on top of the barrel housing you will see a little round thing if you get a small screwdriver and push it down it will release the barrel and you just pull it out.
you still ned the key turned to position 1 while you push the locking tab down like wise to refitting the new barrel turn key to number 1 hope this helps