You can sell your altcoins from airdrop on cryptocurrency exchanges such as Bittrex, Poloniex, Cryptopia or Binance. Get an account on them and transfer your altcoins to that exchanges for selling
Once you see the name of the bursa go there and you can sell your altcoin, if you do not know your coin market try by going to coinmarketcap and searching your altcoin whether registered in coinmarketcap