It’s literally how the constitution is changed. You make an amendment that changes the constitution. If you wanted to change the whole thing in a single amendment, you can do just that.
If you wanted to start from scratch and do the whole constitution over, you’d have the exact same set of steps to do that, unless it was done with an armed overthrow of the government. Then what you would have is a small group of people who ran that revolution would write a new constitution. And that would be unlikely to be any better than what is currently there.
How exactly do you think we would get a new constitution otherwise?