Can someone identify what is wrong with this flower and what should I do to take a better care of it
As you probably realise, millions of these plants (Poinsettia) are grown specially for the Christmas market, usually sold relatively cheaply at many outlets such as supermarkets and also garden centres (where they'll be more expensive!). They will have been 'hot housed' to get them ready for sale, and by that I mean grown in massive greenhouses with perfect lighting, moisture and most particularly, temperature. When you buy your plant, its already been shipped to wherever its going to be sold, and probably exposed to cold air as its moved from delivery truck to the store; then you buy it and expose it to cold air again as you take it to your car. This often means we're lucky if our Poinsettias make it to Christmas Day and still look healthy, so don't feel too bad about what's happened to yours.
They don't appreciate sun, they just like good daylight; they loathe temperature fluctuations, temperatures below 15deg C, draughts (so near windows isn't great), being placed near a heat source, heavy, slow draining soil and dry air. They need to dry out a little between waterings, which means the surface of the soil in the pot should feel just a bit dry at the surface when you water. When you do water, water well, but never leave any outer pot or tray full of water - empty it after 30 minutes so that the plant is not sitting in water.
Now that yours has drooped, it may well not recover, unfortunately - you may find all the green leaves drop off and you're left with just bare stalks with red bracts (the red 'leaves', the flowers are the tiny bud like objects in the middle) at the top, which isn't a terribly attractive look. However, although most people just throw these plants away after Christmas, it is possible to get them growing again and even to produce those red bracts for next year, but it is a bit of a performance. The way to start the process is, in fact, to let your plant dry out so that the leaves wilt and drop off; as yours has already started doing that, it might be worth a try. After the leaves have dropped off (including the red bracts) move it to a cool dry place and keep the soil just slightly moist until spring. Then, once spring arrives, prune it down to 4-6 inches high and move to a warmer, brighter spot. You should see growth beginning, at which point start watering as normal again - treat as a normal houseplant, fertilize every couple of weeks, and then later, around September, you will need to give it 12 hours of complete darkness daily for 8 weeks to get the red bracts to form. Full, detailed instructions here https://www.wyevalegardencentres.co.uk/tips-and-advice_how-to-care-for-poinsettia