1. Turn your Dell D420 laptop over.
2. Find the laptop battery release button lock for your Dell D420 laptop.
3. Now that your Dell D420 laptop battery is unlocked, find the battery release button down.
4. Holding the laptop battery release button, gently pull your battery away from the laptop.
5. Now that your Dell D420 battery is removed, grab your replacement Dell D420 battery for installation.
6. Insert the battery into the slot. Push the Dell D420 battery towards the laptop and secure it in place.