You can use spray a solution of a couple of liquid soap drops + 1 tbsp of baking soda + 1/2 gallon of water on all parts of the shrub including the bottom parts of the leaves.
Dispose of any fallen leaves or plant debris on the trash to prevent spread of the fungi. Warm days, cool nights and a little rain usually get this problem started so weather changes might also help.
Spreading the shrub away from other plants so there is good air flow and watering the soil early in the mornings instead of watering the leaves can help.
Chemical solutions: Some of the fungicides cleared for the control of Powdery Mildew on Hydrangea are azoxystrobin (aka Heritage; has the smallest application rate and largest repeat interval); fenarimol (aka Rubigan); fenarimol (aka Cleary's 3336). You can also use Sunspray Ultrafine Oil.