Trim away any loose rubber seal around the power button with a sharp knife.

You can either leave the tiny plastic button embedded within the seal. Or you can try and remove all the seal around it while managing to leave the button attached by the tiny hard white plastic tab to the main casing.

The power button might turn out to be already broken off and thus will end up completely detached. If so you'll need to encapsulate it carefully in the right position within the new sealant when you reassemble - see step 11.