You can change it by gently prising open the smart key cover (you may need a small tool for this, which sometimes comes with the batteries, although you could just use a small screwdriver instead). It requires a little writing, but you can pop the case apart. Once open, you can swap out the old battery in the battery compartment for the new one and then put the case back together again. Your batteries may come with a video with a tutorial on how to do this. Remember to discard your batteries carefully: they need to be recycled (depending on local services) or taken to household hazardous waste collection points. Batteries shouldn't be thrown away with your normal trash, as they could explode, causing a fire.