Q.Brown hyacinth flowers
I purchased a hyacinth grown in water. The flowers are now brown. What do I do with it?

The forced Hyacinth Bulb has completed it's life cycle. Often times these forced bulbs are enjoyed while they bloom and then can be tossed away, or you could choose to keep the bulb and plant outdoors.
Here are some links that more information for you.
https://www.gardeningknowhow.com/ornamental/bulbs/hyacinth/care-for-hyacinth-houseplants.htm
https://www.gardeningknowhow.com/ornamental/bulbs/hyacinth/browning-hyacinth-plants.htm
https://www.gardeningknowhow.com/ornamental/bulbs/hyacinth/curing-hyacinth-bulbs.htm