You require a CR 2025 3 V cell battery.

Take the mechanical key out of the SmartKey.

Press mechanical key 2 into the opening in the SmartKey in the direction of the arrow until battery tray cover 1 opens.
When doing so, do not hold cover 1 shut.
Remove battery tray cover 1.

Repeatedly tap the SmartKey against your palm until battery 3 falls out.
Insert the new battery with the positive terminal facing upwards. Use a lint-free cloth to do so.
Make sure that the surface of the battery is free from lint, grease and all other forms of contamination.
Insert the front tabs of battery tray cover 1 and then press to close it.
Insert the mechanical key into the SmartKey.
Check the function of all SmartKey buttons on the vehicle.