Get the correct replacement battery for your Honda key fob, usually a CR2032 or CR1620
Locate the seam on the key fob
Insert a small flathead screwdriver or coin into the seam
Twist gently to open 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 key fob buttons
Replace the battery cover if your model has one