You will find it easier to do this with the wheel off and the barrel housing removed.
You will need a piece of tough wire, i used a piece of coat hanger. Bend it so that it has a hook on the end. On the rear of the barrel there is a hole that you will need to locate with the wire.
This in turn depresses a spring that releases the barrel. Sounds easy but it is a little tricky. Look at the new barrel to work out the position of the hole on the the back.
Enough from me use this:
http://www.specialpatrolgroup.co.uk/spooky/barrel/barrel.html
Good luck,
Kev