Destring it. get an iron and gently heat the bridge just enough to soften the glue. Get a spatula and gently slide it under the bridge. This may take a long time. Eventually the bridge will remove.
You need to check if there are rivets on either side of the bridge. There often are with 12 string guitars. Then you need to see if your top is flat or pulling away from the inside. If it is it will need gluing back and clamping. Eventually reattach the bridge when you have sanded off the old glue Clamp it up and when it is dry restring it and off you go.