When you create your account on the mobile app, you will be asked for the country you live in. That will automatically apply default currency for your account.
To change the default currency, please log in to the web app. Then navigate to "Account settings" and open Account details tab:
Otherwise, you can edit the currency for each item on the mobile app from the "Edit" button that appears before submitting.
If you want to correct the currency after the item is processed, simply open the item and update the currency from the menu.