You need the correct blade and a wrench. Use blade part number 3707494002 or 3707130003.
- Disconnect the battery. Tip the mower on its side with the discharge chute down.
- Use a block of wood to jam the blade from turning.
- Remove the prevailing torque hexagon lock nut (part number 5630351002) and washer (part number 5650705002) securing the blade.
- Install the new blade, ensuring the cutting edges face the ground. Reinstall the washer and lock nut, tightening securely.
If that didn't work, you may have the wrong blade. Confirm the part number matches your model. Also check the blade adapter on the motor shaft for damage.
Heads up: Always wear gloves. A sharp blade is a serious hazard. Check tightness after the first few minutes of use.