The main function of the air conditioner filter is to clean the air. This will prevent any debris, mold, dust and allergens from entering your home and causing health problems. After some time, filters tend to accumulate too many debris and thus it becomes ineffective.
First of all you will need to find out where the air conditioner filter is installed. Most of the times it can be found in the air handler, behind an access panel.
Find the access panel and then remove it to handle the air filter. You need to be careful as you will have little space to manipulate the air filter. Try to extract the filter and examine it.
Sometimes the air conditioner is so complicated that you may not find the air filter. The manufacturer may have placed it in the return duct of the unit. To find it, inspect the sheet metal panels in the duct and see if the filter is slid into a track.
To put back a new air filter you need to follow these steps backwards. If you have reusable air filters, just remove them and then wash them with warm, soapy water. Let them fully dry and then put them back.