Hello Irma, Only take off those stems or flowers that are now finished.
this might be helpful: https://www.thespruce.com/when-to-prune-hydrangeas-1403320
My name is Linda and I have hydrangeas. Some hydrangeas are best pruned in late fall, while others should be pruned in the summer. It depends on whether they bloom on old or new wood. If you look on the stalks of those that bloom on new wood, you will see the start of next year's flowers not too far down the stalk, next to a pair of leaves. That type should be pruned in the summer as soon as the new flowers start to fade. Here's an article with more information that should help you figure out which type of plant that you have. Wishing you the best!