Get the correct replacement battery for your Subaru key fob, usually a CR2032 coin cell
Locate the seam or notch on the key fob case
Use a small flathead screwdriver or coin to gently pry the case open
Separate the two halves of the key fob
Remove the old battery
Note the battery orientation before removing it
Insert the new battery in the same orientation
Snap the key fob case back together
Test the buttons to confirm the fob works
Reassemble the mechanical key if it was removed