If they are healthy and growing well, it might be best to follow the advice of leave well enough alone. Otherwise, you have a couple of options. You can scratch in some compost to the top of the soil and it will gradually work in when it rains and when you water. Over several years, this will improve the soil. Or, if they are struggling, this is a good time of year to dig them up, add compost to the existing soil and replant them. You didn’t mention what kind of hydrangeas they are. Some hydrangeas aren’t picky about soil. Some of the bigleaf hydrangeas with colorful flowers will have blue flowers with a fertilizer with aluminum sulfate. Bailey Nurseries has a great website with helpful information on growing blue flowering hydrangeas including Endless Summer. It covers soil prep, planting, fertilizing, watering and pruning.
Thanks for writing!