Thetford cassette door 3 rubber seal

Hello
I need to replace the rubber seal on outside hatch door, it’s the toilet cassette Thetford door 3.
I can find replacement rubber seals online but looking at the old seal it seems to be trapped into the door itself.
Can anyone please explain how to take out existing rubber seal and replace it with a new one.

The problem is that water seems to be getting past the rubber seal.

Hi Julie. It might help if you change your profile to show the model of your van rather than "coachbuilt" then I’m sure someone will have an answer. 

It simply  pulls off  the new seal is pushed back into the grooves    very simple fit  ,

Where has Julie Dr gone, on a callout?

Last online Saturday, yet another new member asking a question, checking fir replies and not having courtesy to thank members who have gone to effort of responding to their request 

Very disheartening

Yes, disheartening, sounds like a lack of enthusiasm for the product or ?lifestyle? of motorhoming. Or perhaps she just hasn't got a round to it yet. If so here's one:
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On the other hand Julie Dr could have recently bought a new phone, which brings lots of associated wonderment. After I recently got a 'new' phone from one of the high street used phones shops I saw a lot of wattasapp messages were sent by people, but not received by me. It was odd. I got notification that Joe Bloggs and Mary Smith had posted wattsapps, but could not access the messages they had posted. All sorted now though.