need help interior of plane

I was wondering ive searched google a thousand times with no answers. how can i redo the interior of and aircraft on fsx. example c172 Cessna i wanna remove the back seats and add small cargo boxes and mail bags. it bothers me a little looking back to an empty plane when im flying assorted cargo on Air Hauler...P.s. if you can just add a url of a similar question that has been answered or tutorial be very grateful thanks......