Identify the key fob type and the battery model it uses
Press the release button and remove the metal emergency key
Use the emergency key or a small flat tool to open the key fob case
Separate the two halves of the fob carefully
Remove the old battery
Note the battery orientation before removing it
Insert the new battery in the same orientation
Make sure the battery is seated securely
Snap the key fob case back together
Reinsert the metal emergency key
Test the key fob buttons and locking functions
Replace the battery with the correct type specified for your Mercedes key fob