Unlike bouquets, the flowers in an arrangement are inserted into floral foam filled with water. Floral foam does the same thing as a flower stem: it takes in and absorbs water. An advantage of using floral foam is that it holds a large amount of water, keeping the flowers moist for longer.
Add a little tap water at room temperature every day or two, depending on the size of your arrangement and the season. It's best to use a small watering can and pour the water from the centre upwards.
Don't add any preservatives, as your florist will have added enough for several days.
Remove the flowers or buds once they have wilted. Unlike a bouquet, you don't need to bevel the stems as the florist has already done this. It is quite normal for flowers to continue to grow and stick out a little.