Where to send the letter about FBI NC over 180 days?

I am married to a US citizen and waiting for my FBI NC approved for my GC. I have read some articles about sending a letter to USCIS because of the new 180-day rule. I'm wondering if anyone did send a letter and heard anything back from USCIS? If I want to send a letter, what's the address? Do I just send the letter to the local USCIS office?
Thank you for your help and good luck to everyone!!!