How and when do I cut back my hydrangea plant. It has grown quite tall

To make bushier plants prune entire plant to about knee high and give a heavy feeding of a long acting balanced fertilizer like osmocote 14-14-14 in early February. If you find any dead branches, break them off or prune them off. Bait for slugs and water often. New fresh growth will arise from the base. Blooms on old wood generally but some varieties bloom on new growth. See “hydrangea plus” for more detailed advise.

There are several types of hydrangeas. I suggest looking it up on line—there are apps that can help you identify the particular hydrangea you have. You can then check reputable sites for directions and timing of pruning YOUR hydrangea.

If if you can’t find out that way, try taking pictures ok&/or a sample to your local garden center and ask a knowledgeable person what you have and what to do. (Then double-check what they tell you on-line. Not every garden center has well-experienced plant staff.)

Here's a helpful post on how and when to prune hydrangeas: https://www.hgtv.com/outdoors/flowers-and-plants/trees-and-shrubs/hydrangea-care-to-prune-or-not-to-prune-your-hydrangeas

Fall is an excellent time to cut them back!