If you have ETH or other tokens that you want to exchange for fiat currency, you will have to do so using an exchange platform, as MetaMask does not provide this service. This means sending the tokens manually to the exchange platform of your choice.
For details on how to send tokens from MetaMask, please check our article on sending tokens out of MetaMask.
NOTE: Always check that the exchange platform supports the token you want to exchange before sending it. Exchanges are also likely to use different addresses depending on the network. Make sure you're sending to the correct one.