How to proceed to sell Bitcoin in the market when the purchase was made in 2010 was a value of 21k USD.
Open an account at a suitable exchange that services people in your area. In the United States, Gemini (now called Coinbase Pro) would be a good choice. You should not be paying fees more than 1% of the prices you see on bitcoin pricing sites like coinmarketcap.
By my math, that would be about 320,000 bitcoins with a current value of approximately $900 million. (You'd have 320K BCH and BSV too, worth about another $80 million.) But it's hard to imagine that this story is accurate because it would have been very difficult to buy so many bitcoins in a single transaction back when the supply was so small.