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Page 2-10 of the 2009 owner's manual. Take the key out first.
1. Separate the transmitter with a flat, thin object
inserted into the slot on the side or back of the
transmitter.
2. Remove the old battery. Do not use a metal object.
3. Insert the new battery, positive side facing down.
Replace with a CR2032 or equivalent battery.
4. Reassemble the transmitter.