To replace the handle, first remove the green rubber cap from the inner release handle. Then remove the screws from within the inner release. Remove the black push rod that connects the inner and outer handle. You'll need a 17mm socket with an extension bar. Place the socket into the hole and find the 17mm nut which is at the back of the external handle. Turn anti-clockwise. There are 2 slotted screws above and below the latch part of the handle. Remove these and the handle will come free from the door. There will remain a brass coloured fixing plate on the door. This can be reused if you like.

To fit the handle repeat the process in reverse. Once fitted, be sure to check the strike and adjust if necessary. You'll need an 8mm spanner for this. It's important to get the door to close as tight as possible without it being over tight and putting to much strain on the strike. You'll have to adjust it a few times and use your best judgement.

Doors can often be twisted and the door won't seal in either the top or bottom corner on the closing edge. You could try placing a wooden block in the corner opposing the gap and apply pressure to the face of the door. This may put the door back into twist and help it seal.