In autumn you just take away the faded blossoms. It depends on the species what needs to be done in spring. There are different species like Hydrangea macrophylla, Hydrangea petiolaris, Hydrangea paniculata, Hydrangea aspera, Hydrangea arborescens or Hydrangea serrata.
Hydrangea macrophylla: In spring, they are just cut back if required. Usually, you do not cut them in winter because they have already formed the buds for the next year. Make sure - if you're cutting in spring - to just cut the faded blossoms back to the latent bud.
Hydrangea petiolaris: In spring, just thin out the branches, if required.
Hydrangea paniculata: They bloom at branches of at least one year growth. Therefore, in spring you can cut them back to 15 cm above the ground. Thin out if you have a bigger shrub and cut it back down to 3 to 5 buds in springtime.
Hydrangea arborescens: Similar to Hydrangea paniculata but a little less strict.
Hydrangea serrata: You do not have to cut them. Just thin them out and take away the old flowers (similar to Hydrangea macrophylla).
You do not have to cut back Hydrangea aspera and Hydrangea serrata. Just thin them out and take away the old blossoms (similar to Hydrangea macrophylla).