Unscrew the lug nuts all the way.
Since you've already loosened them, you should be able to unscrew them mostly by hand.
Remove the flat tire by gripping the tire by the treads, pull it gently toward you until it's completely free from the hub behind it.
Set it on its side so that it doesn't roll away.
While you have the tire off and if it is safe to do so, take a minute to check your brakes, rotors and any other component that it is usually hidden by the tire.