Wait until after it has flowered before pruning, otherwise you will lose all this years flowers. Then cut out, right at the base about 1/3 of the shoots from the middle of the bush. Do this every year for three years and you will have removed all of the old branches and replaced them with new growth. Thereafter remove all the flowerd branches after flowering.
OR. You could just take a saw and cut across about 18 inches from the ground, lose all this years flowers, feed the bush and wait for next year!

thanks