Below is a guide on how to fit a pvc window
Note : The less damage you do taken out the old window the less work you have in making the new one good.
Measure your new window to make sure it will fit
Than run a sharp knife around the inside of the window frame to break the plaster seal to the window
Cut out the mullion and transom bars with a saw
Than lift upwards to break it at the corners
Wedge out the sides slowly with a chisel or bar
If there is some break them with the chisel
Than pull the side upwards to break from head
Once your window is sitting fully on the cill
You than slide your window so that it is behind both reveals
Plus now clip on the drip for bottom of the window this will now extend your window so that it steps around the cill
Plus you can also pack a sand and cement mix under the window to fully seal your new window to the cill
The inside can be finished with some patching plaster If some of the old plaster broke away