Heat for a few minutes om very low heat in the oven (I used 50C). When the sealant starts to become soft to the touch it's ready to be scraped off, and the lens gently and patiently prized off.
Not difficult at all but a bit of a time consuming bit of work with patience involved.
Then mask off both the outer most part of the lens so you don't get sealant all over it and mask off the part of the housing outside of the place for the sealant. Fill up the gap again with some black bathroom sealant an overdo it. That way it will overflow and you'll be sure not to have any air pockets giving you condensation.
The sealant that now got squeezed out will hopefully only be on the masking tape and will be a breeze to remove for a nice and clean lense install.