Certified GKH Gardening Expert
The leaves are turning yellow due to the fact that it is trying to go into dormancy. Hydrangea really need to go into dormancy for a few months to be healthy. Do you have an attached garage or basement - basically somewhere relatively cool and dark - you can place it in for a few months to do that? Once there, it will lose all its leaves. Water it once or twice a month and bring it back out in the spring.