You have to wait till the coin of your ICO to hit the exchanges and trade your ICO coin to BTC. After that use an exchange (eg. coinbase) to sell BTC for fiat.
Alternatively, you can post in marketplace to exchange your ICO coin for fiat with another member here. Remember to use escrow for such trades.
Thank you for your response, but I think you're missing my point..
If
MY ICO was successful and I have $40 million how can I withdraw this to fiat currency?
You need to go to an exchange where you can trade your ICO token/coin with Bitcoin or Ethereum. Exchange them with Bitcoin, Litecoin or Ethereum. Then go to Coinbase or other place exchanging them with real money. And then send them to your bank account.
The process is complicate...