Just changing to a 15,000 btu will make a huge difference. Swapping out the units is relatively easy, hardest part is lifting the unit to the roof and I don't suggest you attempt this by yourself. Everything is pretty much plug and play, no new wiring needed. If you also add the rotating vent to the inside cover you should have plenty of cool air. About $700 for a new unit, sell your old unit, do the work yourself, should easily have less than $500 in the swap.
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