Depending on the wallet you have or the exchange you use, it’s possible to exchange your BCH for BTC, and vice versa.
Some exchanges and wallets, however, require you to first convert your coins to a centralized currency, like the U.S. dollar, and then convert that amount of currency into the new cryptocurrency. However, there are exchanges that will let you convert from one coin to another. For example, Coinbase and CoinSwitch allow for direct conversions between BCH and BTC.