Lift the latches at the top of the vent and move them to the left. This loosens the top of the air in-take vent. Remove the vent cover and set it aside.
Remove the air filter. The size of the filter is printed on the side. Take your filter to the hardware store to ensure that you have the correct size.
Line up the new filter with the vent hole. On top of the filter there is an arrow that shows how the air should flow through the filter. Point the arrow toward the inside of the vent. Push the filter into the space.
Replace the vent cover over the air vent and push the latches back at the top. The latches must lie flush with the wall to remain latched.