You can change the source of the price for each coin in your Portfolio or Watchlist. There's two ways to do that:
Open the coin card and above the charts, you will see the source exchange and pair displayed. You can tap this box to select the exchange and trading pair you'd like to view prices from. The chart will also update to reflect the new source.
Open the coin card and tap on the 3 dots to the top-right to open the options menu. Here you can select the exchange and pair you'd like to view the prices from. The chart will also update to reflect the new selection.
Global average selects coinmarketcap.com as the source (and is located at the very top of the list of exchanges)
In the above example, if you select Binance and ETH/BTC as the source for Ethereum, the app will show the ETH/BTC graph with prices sourced from Binance. The price is converted using the exchange's BTC/USD or BTC/USDT rate to display the price per coin in USD (or your default currency). If the exchange selected does not have a BTC/USD(T) pair then the BTC/USD from coinmarketcap.com is used. You can tap on the ETH price in USD to interconvert between USD and the pair selected (BTC in this case).