Dilute it using water to lower the concentration. Use the formula #c_iV_i = c_2V_2.# Assuming you have 1L of 0.40 M solution, you can solve for #V_2=(c_1V_1)/c_2 = ((0.40)(1))/(0.20) = 2 L# (Remember #V# is always measured in litres.) Notice that this is two times the initial amount of #1L#. You can deduce that to reduce the concentration or molarity of a solution from 0.40 M to 0.20 M, you must fill the solution with a volume of water that is equal to the volume of solution.
Good luck :).