You change the battery in a Mercedes key fob by pressing the release button and sliding out the emergency metal key blade. Use the mechanical key tip to push the internal release tab and pop open the battery compartment cover. Remove the old coin cell battery, typically a CR2025 size, and insert a new battery with the positive side facing upward. Snap the plastic cover back into place securely and test the remote buttons to confirm functionality. This simple maintenance task takes only a couple of minutes.