One of the interesting differences between American cars and European cars is the location of the stock battery. On most American cars, it's under the bonnet, whereas on most European cars, it's behind the driver's seat either in the boot or under the seat. In a Mercedes-Benz E-320, it's under the rear seat. Once you figure out how to get to the battery, removing it is straightforward.
Open both rear doors. Look at the base of the seat and look at the area where the bottom of the seat and the floorboards connect. There is one latch on each side. Pull the latch to pull up the bottom of the seat and pull it out of the car.
Unbolt the battery hold-down clamp at the front of the battery using the 3/8-inch ratchet and socket. Then slide the battery toward the front of the car.
Disconnect the negative terminal on the battery with an open-end wrench. Then pull up the plastic cover on the positive terminal and disconnect that connection with the wrench as well. Pull out the battery, and pull off the plastic battery vent hose as well.
Set the replacement battery in the stock location and plug in the battery vent. Reconnect the battery terminals, installing first the positive then the negative terminals, then tighten the hold-down clamp to hold the battery in place. Put the seat back into the car and latch it in place.