The first thing you should do is really clean up the stem by peeling away the foliage, either with your hands or with your clippers so that there’s nothing below the water line. Cut the stem at a 45 degree angle using using clippers or knife. This increases the surface area on the bottom of the stem so that the water can travel up to the bloom. If you wanted to freshen the hydrangea that’s starting to wilt or dehydrate, you can also spray it with a spray bottle full of nice cool water.