Simple, move forces from the Maginot Line (while still leaving a sufficient force there to ensure it isn't undefended enough for the Germans to change their plans), and change battle plans accordingly, all while trying not to let the Germans know their secrets have been spilled.
France was in many ways more developed and powerful than Germany, and the German attack relied on surprise, outmaneuvering, and air superiority to succeed. Take the first two away, and the Germans are in for a tough battle. They likely get bogged down, the situation becomes kind of like the stalemate seen in World War I, and assuming Hitler is stupid enough to invade the Soviet Union and declare war on the United States anyway, the war ends a year or two early.