Get the correct replacement battery for your Subaru key fob, usually a CR2032 coin battery
Locate the seam or notch on the key fob
Use a small flathead screwdriver or coin to gently pry the fob 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 halves back together
Test the buttons to make sure the key fob works
If the fob does not work, reopen it and check the battery orientation and fit