When you create an account, you'll be asked which currency the business regularly operates in. That will be the default currency for the 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 while submitting.
If you want to correct the currency after the item is processed, simply open the item and update the currency from the menu.