1. Turn your Compaq 2521EU laptop over.
2. Find the laptop battery release button lock for your Compaq 2521EU laptop.
3. Now that your Compaq 2521EU 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 Compaq 2521EU battery is removed, grab your replacement Compaq 2521EU battery for installation.
6. Insert the battery into the slot. Push the Compaq 2521EU battery towards the laptop and secure it in place.