I changed the filter on our central heating and air conditioning unit today. The manufacturers recommend the filters be changed monthly. I check mine monthly, but only change it when it is dirty.
Our is very easy to change, it just slides in and out. If you have never changed yours, you might want to check with your favorite mechanically minded person for a little help.
Using a clean filter will help your unit run more efficently and keep your home's air fresher.
In the picture, the old dirty filter is on the left and the clean new one is on the right.
About a year ago, I found a batch of filters at the thrift store that fit our unit for just 25 cents each. Sadly, we only have 1 of them left.
Filters can be bought at any home store or big discount store. The sizes vary so check yours before you buy new ones,