I see ligth from the sides of my iphone 5s and warranty want replace it because it ended only 20 days ago ??? help

What difference would it make if they did? There are previous posts here you can search for where people had light showing but they immediately sought service when they spotted it and had it replaced. How long ago did you notice this?

If you were out of warranty when you noticed it then that is too bad but you can do an out-of-warranty replacement.