If you replace with a like unit *Same make and model* you have the easiest job. Remove ceiling (intake) grill and filter. Unplug an umbicial or two, remove 4 bolts.. go topside (Take new Roof unit with you it's about 90 pounds)
Remove roof top unit and gasket
Clean
Lay down new gasket
Lay roof top unit on top
Partner hollers directions so it lines up
4 Bolts
A plug or two
Replace intake
Done
If you change makes you may need to replace the control electronics and the lower unit. the only thing you likely will NOT need to replace is the center unit (The ducting part) Go ith a 15K unit not a 13.5.