It is very easy to change. The cabin filter is located behind the glovebox door. You need to remove one Philips head screw (on the right hand side) then squeeze or compress the sides of the glovebox until you can drop the tabs that stop it from dropping to the floor. Let it hang by its hinge and remove the old filter.
The last time I checked, 8 months ago, the auto stores did not carry them. My dealer wants $29 a pop, so I make mine out of the HomeDepot Mod
Eighteen months is way too long of an interval. Yours will look like the ones posted. Yucky.