When you receive a flower arrangement, first make sure there is enough water in the container, especially if the flowers are arranged in Oasis (green foam). Take a photo of the flowers, a close-up, if possible. Place them in an area where they will not be subject to cold or hot drafts and direct sunlight. Add water as needed (usually once per day). Some flowers drink up water more quickly than others, so check the arrangement several times.
If, on the next day, the flowers look wilted or have "bent-neck", call the florist as soon as possible, to lodge a complaint. But first, take another photo of the arrangement, showing the problem. That way, you will have a basis for comparison, and can offer to email both photos to the shop.
The shop should give you an immediate replacement, or a credit. If the flowers were a gift, you have to decide if you want to let the sender know that they were not good quality. Most people won't want to tell the sender, but I think you should, so they can complain and receive a credit. They might be embarrassed or mad about it, but they are certainly not to blame - especially if the flowers were sent from a distance and the sender had no way to judge the quality of the florist shop. Otherwise, they will think you were pleased, and will call that same shop for future deliveries.
Over the course of my career, I have worked for 7-8 different shops; most of which had a liberal credit policy, especially in the face of poor quality. Florists cannot always tell when flowers they receive from the wholesaler/shipper are not good quality. They expect to be called out for delivering poor quality flowers and in many cases, they can receive a credit for their purchase (or may already have).
The few florists I know who send out poor quality flowers and/or refuse to repair a problem like this are allowed to remain in business because people do not complain promptly or tell others about their experiences. You are doing everyone a favor when you don't let them get away with it!!