I've taken out a sash window to mend the cord - you have to lever out the three pieces of wood which hold the moving window frame in place. The wood is only held in place be small nails and should be easy to lever. Now that you can get to the frame you can swing it inwards and detach the cord from each side. If you want to remove the top window as well you have to remove the long strip of wood either side of the sash frame that holds the the two windows apart. This should, once again, just lever (or pull) out. The top window cords can be removed as you did with the lower ones.
It sounds complicated, but it isn't really. They are very simple things. But if you're worried, do contact a proper chippie. This site -
http://www.square-sun.co.uk/sash-style/re-cord.htm
,
tells you how to re-cord your window, so gives you an idea about how to get the window out in the first place. Good luck!