You should change your engine air filter every 15k-30k miles depending on your driving conditions. More often if you commonly drive in dusty areas or dirt roads. If you do not drive a lot of miles then it should be changed every 3 years, they become brittle with age and can start to come apart.
On most vehicles the cabin air filter is behind your glove box and are usually pretty easy to reach. After removing contents of glove box, unhook the little arm that supports the glove box when it is open, usually this is on the right side of the glove box. Behind the glove box you may see a face plate