To switch to a different exchange or currency for a coin in your watchlist do the following:

Step 1:
Select the coin in the watchlist OR go to to the quote screen. (The quote screen shows information such a price, volume, high, low, open, market cap, etc.)

Step 2: Tap the blue link that shows the current exchange and currency (for example, 'Mkt Agg - USD' and simply select your preferred exchange-currency combination.