I would expect that if you want to invest your bitcoins that you would have to change them to cash and then you can buy stocks with that cash. Or, if the return is worthwhile, invest it into more hardware and whatnot for making more bitcoins, but it sounds like the return isn't very good.
Why should he invest in stock market dump ass if he already has cryptocurrencies , you want him to transfer bitcoin into fiat currencies , how stupid you are , our crypto world need more people to invest in crypto not to get the money out and buy stocks . As for you OP you have only to create an account in one of the cryptocurrencies exchange platforms like binance , transfer your bitcoins to the platform , start making research about the coins with good potential to invest your money in , you can find good informations here in this forum on altcoins section , on YouTube channels , Twitter or telegram . Consider that it could take some time to find the good investment , try in the beginning to take advice from experts . This is my advice for you .