Stop down the lens to permit a depth of field that will get more of the blooms in focus.
I do not think it is under exposed. I would bring down the highlights. Bring up the shadows. Then I would sharpen the flower.
You need to retake the picture with correct exposure, focus on the blossom. Then you need to carefully edit the picture to work out the flaws, without destroying the detail.
Here you simply did apply aggressive de-noising followed by excessive sharpening.
The out of focus background (depth of field) should stay soft and smooth. And just to say: the out of focus area should come from the optics, not from the post-processing...
I'm not convinced that the picture is under exposed. I think it's missing contrast and could get a small boost of vibrancy. Enhancing local contrasts (texture and clarity) could help also.
@Marta1982, The framing of your photo is not ideal. The flowers extend beyond the top and bottom edges and everything has the same depth of field. Also, there is nothing else in the image to give a sense of scale. Are these small or large flowers? Think about “settings” when you take images of flowers like this in a bunch. Are they near a beautiful walking path? In front of a cute cottage? By an old fence? Near some animals? In a window box? Anything to give the viewer a measure or guide helps to make the photo more interesting. If you were aiming to take a specimen image, the blossoms (are they echinops?) seem past their peak. Photos of blossoms should be taken at their most optional moment. Hope this feedback is helpful. Keep shooting!