How can improve the quality of its selfi camera?

You'll have to clarify your question, Natan. Are you saying the camera isn't performing as well as it used to, or you're not satisfied with the quality of pictures it produces as it came from the factory?

If it seems like there's something wrong with it, you could try removing any screen protector or case you might have and clean the screen.

If the pictures just aren't good enough for you, I'm afraid the only real fix is to buy a phone with a better camera. There really is no upgrade path for internal phone parts such as cameras; they're generally custom made for a particular model and I've never seen a phone that offered options between good and better cameras.