To nevíš, jak se píše for cyklus?

    public partial class Form1 : Form
    {
        readonly Random _rand = new Random();
        List<Rectangle> _rects;
        
        public Form1()
        {
            InitializeComponent();
        }

        void Form1_Load(object sender, EventArgs e)
        {
            _rects = new List<Rectangle>
            {
                new Rectangle(10, 10, 10, 10),
                new Rectangle(30, 10, 10, 10)
            };
        }

        void panel1_Paint(object sender, PaintEventArgs e)
        {
            var brush = new SolidBrush(Color.Red);

            foreach (var rect in _rects)
            {
                e.Graphics.FillRectangle(brush, rect);
            }
        }

        void timer1_Tick(object sender, EventArgs e)
        {
            for (var index = 0; index < _rects.Count; index++)
            {
                var rect = _rects[index];
                rect.Y += _rand.Next(10);
                _rects[index] = rect;
            }

            Refresh();
        }
    }