Get the correct replacement battery for your Nissan key fob, usually a CR2032 coin battery
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
Insert the new battery in the same orientation
Snap the key fob halves back together
Test the key fob buttons
Reassemble the key if it has a separate metal key insert