My friends keep putting themselve down saying they are ugly or cant sing. no matter how many times?

I have had a similar problem in the past with a few of my good friends. My advice to you is that many people may be going through a rough time in their life, and feel insecure. Or maybe they just want reassurance. In my experience just tell them, instead of saying they are better than you or anything like that, is to just share your opinions and if they disagree don't argue, they probably just feel insecure and want to change the subject. The best way I have dealt with this in the past is to try and boost their confidence. Try to show them the best version of them that you have always seen.