I have an EOS key pair from the ICO. Keys were registered at mainnet launch. How do I convert those keys to EOS tokens that I can trade on an exchange?
Download a wallet like Scatter, SimplEOS or Greymass wallet, import your private key into the wallet, then use the wallet to send your coins to an exchange. Simple as that. If you want to know where/how to send them to an exchange, look on your exchange website on how to deposit EOS.
Maybe somewhere in this process you will need to enter your EOS account name, you can find this on https://eosauthority.com/ if you enter your ETH public key (that you used for the ICO) or EOS public key.