making a website designed for mobile suitable for desktop

we have a website designed for mobile and its appearance and operation is awful on desktop. I want to make a change to the css or javascript/jquery of the website so that it will be rendered good on desktop. for example by something like this:

@media only screen and (max-width: 500px) {
  body {
    width: 500px;
    background-image: * ;
  }
}

One of the problems to be solved is that the vw's used to make the display responsive for different mobile sizes would be so big on desktop that would mess everything. please help thank in advance