If your bitcoins are on an crypto-exchange you need to do a sell BTC to a stable coins - USDT,USDC, and then attempt to convert USDT,USDC… to fiat USD.
If the exchange does not do conversion to fiat you need to create a Tether , Circle … etc. wallet and withdraw from crypto exchange to wallet address.
Then you can attempt to convert to fiat.
Currently there are few requests to go back to fiat.
However if the crypto exchange of a stable coin issuer is faced with multiple requests to convert to fiat they will most likely refuse to do conversion, or break the 1:1 peg .