Unscrew the inside fitting first and remove.
Using a ladder get acess to the outside of the roof light. It will be held in place by mastic. slice through the masic and clean the area surrounding the resultant hole.
Put ample mastic on the new outer part where it sits on the roof, it is essentioal to get a good waterproof fitting.
The from the inside screw on the interior fitting with screen(s). This should pull the outer fitting snugly to the roof improving the seal.
Last job is to clean off any surplus mastic which has been squeezed out.
Devonidiot
October 2009