A key fob battery almost never outlasts the life of the vehicle, and will require replacement at some point. Replacing the battery in most fobs takes only a few minutes and requires no training. Mercedes key fobs can be very expensive to replace, so it is usually better to install a new battery rather than get a new fob.
Open the battery compartment on the fob. Many key fobs for Mercedes have a battery compartment on the end. Push it off to open. Some fobs have the compartment on the back. Open this kind with a standard screwdriver or a coin. Insert the edge into the slot and turn counter-clockwise. Some newer Mercedes key fobs require you to release the key from the fob, then pop open the battery compartment inside the keyhole.
Identify which battery you need. The identification number should be on the surface of the battery, such as CR2025. Purchase an identical battery.
Insert the new battery, ensuring you have it facing the right way. The batteries are clearly marked with a "+" on one side and a "-" on the other. Installing it the wrong way will prevent the fob from working.
Test the new battery by trying to unlock your car via the key fob.
If you are having trouble finding the battery, take the fob to a dealer. They will open it for you, usually without charge. Refer to your owner's manual for specific information regarding your key fob.