Each Leaf comes with a battery replacement tool that you will need to change the battery in your Leaf when the time comes.
Once you have the battery replacement tool (or a Torx T3 screwdriver) and a CR2032 coin cell battery, take a look at the tutorial below.
Once the battery is replaced, the Bellabeat app might still show a low battery warning, but this shouldn't worry you. All you need to do is sync your Leaf a few times, and the warning should go away.