How do you install an air conditioner filter?

It depends if it is a central unit or a window unit. A central unit will have a return air duct with a hinged grate. open the grate and pull out the old filter. Check the size and go to the store and buy that size filter. Put the filter in the way you took it out.

A window air conditioner either has a foam, fiber, or screen type filter. If it has a screen type, just pull on the tab that should say filter on it and slide it out. You can was this type in the sink, let it dry and then put it back in. The foam and fiber type you have to remove the front of the unit, usually held on by metal clips behind the facing. Pull the front off and pull the filter out. These have to be replaced