They're not batteries and you shouldn't try to change them yourself.
Your AC uses a lot of power and regularly exceeds the capabilities of your home's wiring to kickstart the motors that run the unit. The capacitor helps to compensate for this by providing a large jolt of electricity that gets the motor started and letting it continue running while storing more for when you need it next.
There are two types of capacitors used in an air conditioning system: the start capacitor and the run capacitor. Most units have both, but there are some that only use one.
