If the air conditioner was replaced for your personal property, it is not deductible on your taxes.

If the air conditioner was for a rental property then you set it up as an asset for the property and take a deduction over several years.

Go to Rental Properties and Royalties.  When you get to Review You Rental Summary choose

• The asset/depreciation section - select start
  • Select "yes" to "Do you have assets for this property that can be depreciated?
  • You will enter the Air Conditioner as an asset  This is where your enter all your rental assets (including capital improvements) 

 You will need to have the amount you paid for the Air Conditioner and the date when it was put into service.

Note that you can claim Section 179 deduction e.g., 100% deducgtion in year placed in service (subject to the $1M Sec 179 limitation ) for replacement HVAC systems installed on commercial rental properties but not for residential rental properties.  Residential rentals require deprecation over 27.5 years.

The disparate treatment is because of an oversight in the drafting of the language TCJA that Congress has yet to remedy.