You change a Mercedes key battery by pressing the release button and sliding the emergency metal key blade out of the fob housing. Use the mechanical key tip to press the internal latch and pop open the battery compartment cover. Take out the old coin cell battery, usually a CR2025 size, and insert a new battery with the positive side facing upward. Snap the plastic cover back together tightly and test the remote lock buttons to ensure it works. This quick procedure takes only a few moments to finish.