Restated another way, the question is how to update an existing item in a collection using LINQ:
List toys = new List();
toys.Add( new Toy() { Id = 1, Color = "Red", Price = 1.20M } );
toys.Add( new Toy() { Id = 2, Color = "Red", Price = 1.20M } );
toys.Add( new Toy() { Id = 3, Color = "Red", Price = 1.20M } );
toys.Single( o => o.Id == 2 ).Color = "Blue";
toys.Single( o => o.Id == 2 ).Price = 3.99M;
toys.ForEach( o => Console.WriteLine( $"{o.Id} {o.Color} {o.Price}" ) );