Your 2003 Land Rover uses an ignition key with an electronic transmitter in the remote. As a theft deterrent system, the remote transmits a signal to the truck's computer that the key in the ignition belongs to the vehicle. The system operates on batteries within the key remote, and provides you with a low battery warning to prevent the batteries from failing. The remote uses two CR2025 batteries, similar to watch batteries. Buy them at most major retailers.
Use a small, flat-head screwdriver or a coin to turn the battery cover anticlockwise. Line up the arrows on the battery cover and the key then lift the cover off the back of the key.
Lift the battery case and o-ring out of the key. You may need a small, flat-head screwdriver to lift it enough to remove it. Pull the two batteries out of the case.
Insert the new batteries into the battery case. Insert the battery case and o-ring in the battery area of the key. Replace the cover with the arrows lined up and turn it clockwise to lock it in place.
Insert the key into the door lock. Lock the door then unlock it. Remove the key from the door and press any button on your key remote to complete the resynchronization.
Change the batteries before they fail completely. You will be unable to start your truck if the batteries fail completely.