To give a better idea on my comments. Creating a class level variable is like this:
Notice that sCar
is declared outside the method, but within the class.
Carro sCar = new Carro();
private void listCarros_SelectionChanged(object sender, System.Windows.Controls.SelectionChangedEventArgs e)
{
ListBox listBox = sender as ListBox;
if (listBox != null && listBox.SelectedItem != null)
{
sCar = (Carro)listBox.SelectedItem;
...
private void btnEditCar_Click(object sender, EventArgs e)
{
sCar.ProperyYouWantToChange = "Stuff I want to change"
}