Gather a small flathead screwdriver or coin and a replacement battery
Identify the battery type in your Subaru key fob, usually CR2032
Remove the mechanical key if your fob has one
Use the flathead screwdriver or coin to gently pry open the key fob case
Separate the two halves of the 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
Reinsert the mechanical key if removed
Test the key fob buttons to confirm it works