Without going downstairs and looking at a gun, you remove the butt which gives you access to the mainspring. You will probably need a bent screwdriver or a pair of pliers to remove the screw which secures it. Once out, that removes the tension from the lower tang/trigger plate.
Unless I am mistaken there are either two through screws or 4 short screws located at the bottom front and back of the frame which keep the lower tang/trigger plate in the frame. Remove them and you can pull the trigger plate/tang/trigger guard out. That gives you access to the trigger spring.
You can make a new spring from drill rod if you prefer not to alter the original.
cheers mooncoon