BNB is the "binance token" that works with the binance smart chain. So you can trade it on binance apart from other exchanges.
Binance
In binance they have the BNB/ETH pair in the spot trading or binance convert section.
If instead of binance you want to change it in another shitty exchange like kucoin, or bithumb, maybe you will have to change it first for USDT or BTC and then convert again for eth.
Here you can see all the options where it is listed:
https://coinmarketcap.com/currencies/bnb/markets/