Use a small screw driver or even a dime. Look for a small indent in the bottom of the remote between the two halves. Some remotes have this indent, others do not. Use the screw driver to pry the top and bottom apart.
If there is no indent, then work the tip of the tool along the edge to separate the top and bottom. Be careful removing the battery, as on some models the small metal holder will break off the circuit board and you will need to get a new FOB. This is the most common failure on these remotes.
See picture below.
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