2003 Citroen Berlingo key fob.
1. Pry it open with a flat-headed screwdriver.
2. Push the battery in the direction of the red arrow. You will need a CR 2016 battery for replacement.
This picture shows where the RFID tag is on the PCB. There is an antenna near the ignition-switch which "wakes up" the tag and reads it. The information from the RFID is used for the cars immobilizer system.