Mercedes-Benz 2008 M-Class Operator's Manual page 463

Practical hints
Replacing SmartKey batteries
3 Batteries
4 Contact spring
462
Pull out batteries 3.
Using a line-free cloth, insert new
batteries 3 under contact springs 4
with the positive terminal (+) side
facing up.
Return battery compartment 2 into
housing until it locks into place.
Slide mechanical key 1 back into the
SmartKey or SmartKey with
KEYLESS-GO*.
Check the operation of the SmartKey
or SmartKey with KEYLESS-GO*.