You can either wait a week or so for the ootheca to completely harden and then carefully pry it off or even more carefully cut it off, or you can put that cage outdoors in some sheltered location out of the sun and rain where it can experience the change in temperatures that will send it into diapause. You'll have to keep it outdoors until it hatches on its own in the spring if you decide to put the entire cage outside.
If you decide to remove the ootheca from the container, you can just pry it off when it's hardened by carefully grabbing it as close to where it's stuck to the container as you can and just pulling it off. If you decide to cut it off, you'll need to be extremely careful and use a blade and scrape it off without cutting into the ootheca. Once removed, you can put it outside or keep it in the crisper in your fridge until spring. To make it so the mantises hatch out unimpeded, you'll have to glue it to a stick or some other surface in the position it was laid.
I would suggest leaving the ootheca in the container and putting it outside as there are risks to removing it that include damaging the ootheca, damaging the eggs, and allowing fungus and other things to get inside the ootheca.