The Hyundai Elantra key fobs take a CR2032 replacement battery. The Elantra is a sedan, one of the oldest of the Hyundai models. For other Hyundai models see the Hyundai Fob Battery page.
How to replace the Elantra Flip Key (2012-2013) battery
Here’s a video showing how to replace the CR2032 battery in the Flip Key, non-smart key fob of the 5th generation Elantra. This does require a small precision screwdriver set to accomplish the replacement easily: