Like when a girl )or guy) gets a haircut, and goes from long hair to short hair, for example. While it does look nice short, how do you tell them it looks better when it's long, or at least you prefer it long, without them getting offended? This isn't a girlfriend/boyfriend either, just a friend who hypothetically asks your opinion.
if they ask your opinion, just tell them you think their hair looks good how it is, but you think it suits them better longer, or shorter, or whatever. if they don't ask for an opinion, that could come off as a little rude, depending on your relationship with the person and the context and everything.
I don't know how to tell guys their haircut isn't that attractive, but the way to tell girls is just to say, "Your hair was awesome when it was ________!" And if they ask "So you don't think it looks good now?" Say "No, but it was REALLY nice when it was _________." Don't offend girls, the words stick with them forever.