It is an easy swap. The hard part is getting the new unit on the roof.
Place some plywood up top to work from.
Remove the interior cover and remove the 4 hold-down bolts,
Disconnect the A/C unit at the breaker first. Then disconnect the wires at the unit.
Next on the roof roll the unit onto its side and then get it off the roof.
Get the new unit on the roof.
Lean the unit on one side and install the new gasket on the bottom of the new unit.
Clean the gasket area on the roof.
DO NOT use any caulk on the gaskets.
Place the unit over the hole.
Inside now, and center and square up the unit to the opening.
Install the retaining plate and the 4 bolts.
Tighten the 4 bolts evenly and compress the gasket to 1/2 of its original thickness.
Reconnect the power wires to the new unit,
Check the the baffle between the inlet and outlet air is sealed properly.
Reinstall the interior cover and trim.
Turn on the breaker and then turn on the new A/C unit.
The 20 amp breaker and wire is suitable for the 15000 BTUH unit.
Ken