If you have board coins, then you can sell them at any time and receive in return the amount of MAIN tokens that corresponds to the current coin price curve.
To sell coins, go to the page of the relevant board and click the Buy coins button. In the window that opens, select the Sell tab, enter the number of coins you want to sell in the input field, and click the Sell button. In the opened window of your wallet, confirm and sign the sale transaction.
If you are interacting with coin contracts for the first time, then before selling you will also need to connect your wallet to the site and sign a transaction confirming the interaction with the coin contract.
The withdrawal of board coins from your wallet and the receipt of MAIN tokens to it will occur automatically after the transaction is completed.