There are many ways to rehydrate butterflies. The method below is the one I use and find works well for me every time. It is best to practice first on some cheap or damaged butterflies before you try it on something you've paid a lot of money for.

Fill a dish with boiling water. Dip the antennae into the hot water for a few seconds. Be careful not to wet the wings.

Using a small syringe, inject the thorax with hot water. I usually keep injecting until water has dribbled out for a few seconds. Be careful not to inject water into the space between the wings.

Place the specimen on a paper towel while you do any other specimens you have.

Have a shallow plastic container already prepared for the specimens. Put a few layers of paper towels inside the container and spray each layer with water until damp (not soaking wet).