Is it okay to trim hydrangeas in early spring and how far back?

wait until the new growth starts to emerge to disguinish the dead growth that needs to be pruned out

You need to know what type of hydrangea you have to know if it blooms from old or new wood. That says when you can safely trim and still get flowers each season. Like Peggy said, wait until the plant buds out to find the dead wood and trim the dead wood only until you know more about the particular one you have. Take a picture of the flowers, so you can see the shape of the head, and also a close up of the leaves and a picture of the whole shrub and see if a local greenhouse can ID which one you have and go from their on how to trim the shrubs.

I trim mine after they are finished blooming. I cut mine back to about 4 inches.