The BEST time to transplant a hydrangea is after the leaves drop in the late fall. You can transplant it now if you prepare a rich planting hole with organic amendments followed up with lots and lots of water. Hydrangeas love their water. Do it quick while disturbing the rootball as little as possible. Then lots of water.

Thankyou very much.

as it is in a pot you can do it now. dig the hole 1 1/2 to 2 x bigger than the pot dimension. mix the soil you have dug out with potting compost to help loosen it and provide a finer medium for new roots to grow.

carefully remove the plant and if the roots are tightly coiled around carefully tease them loose. Put the plant in so the soil level in the pot is the same as at the ground, back fill and firm in.

if your soil is very dry add a couple of buckets of water into the planting hole allow to drain before putting the plant in.

water well through out any dry spells we may have.

Thankyou very much indeed.