Pry off the battens on both sides of the damaged boards with a flat pry bar. If water has damaged the sheathing, remove enough boards and battens so you can cut out the sheathing with a circular saw set to the thickness of the sheathing.
Nail the replacement boards in place and caulk the joints between the new and old boards. Prime and replace the battens in the same fashion. Repaint the entire section with high-quality exterior latex paint.