How/where do I deduct $6,800 for new windows in rental apartment?

I'm not sure I understand you question completely. If you a renting the apartment to someone else, the addition of windows would be listed as an improvement and depreciated as a separate asset.

Let me know if that helps.

Yes renting to someone else. OK, an improvement and depreciated. No way to take it as an expense I assume?  Thank you.

Unfortunately, no. The De Minimus Safe Harbor exclusion (where you can expense items normally depreciated) is limited to $2,500.

Much appreciated.

You are welcome. 🙂 I will summarize in answer below.

The addition of windows to a rental property would be categorized as an improvement and be depreciated over the useful life of the rental unit.

See page 5 of IRS pub 527

https://www.irs.gov/pub/irs-pdf/p527.pdf

What is you add windows to rental and sell rental within the same year?  How do you add asset to depreciate and end of life within same year?

Those windows can depreciate even over a period of months as long as you add a date you put those windows in service(typically when they are fully installed and paid for) and the date you sold the property.