On iOS: On the search screen, tap the gear icon below the To and From search fields. Then select your desired currency from the list that appears.
On Android: Tap on profile tab in the top right corner. Then tap on the Settings gear in the bottom right. Finally, tap on Currency and select desired currency.
Changing your currency in the app simply changes the the currency in which prices are displayed and will not affect the currency in which your transaction will actually be charged.
Please note that Hopper currently charges bookings in USD or CAD, depending on the origin airport of your trip.
The specific currency in which you will be charged will be noted on the payment screen and on your receipt.