Since the flowers are going over, you can deadhead them and repot now. That looks like quite a lot of hydrangea for not very much pot :-) And maybe not surprising since those sold by nurseries, florists or grocery stores in flower now and at a young age have been forced or chemically induced to do so and are marketed or intended as disposable gift plants.

But they don't have to be. An appropriately larger container, some high quality potting soil and a placement outside but out of direct afternoon sun should revive it.

Hydrangeas appreciate a lot of water so check soil moisture daily. If you want blue flowers, acidify your irrigation water with some vinegar or use a fertilizer for acid lovers (like Miracid). Depending on the specific cultivar, it may not produce any more buds or blooms this season.

You will probably want to provide winter protection if being left on an exposed balcony over winter. Bubble wrap around the container and some garden fleece or a towel over the stems should work.