If you are using a Money field in your app, the default currency might not be the one you wish to use. You can change this to just about any other currency via the app template.
Simply modify your app template and find the Money field. From here you'll see a box where you can select a new currency.
Want to change the currency in a report?
You can do this too! Simply edit the report by hovering your mouse over the tile and clicking the wrench icon that appears, then "Edit". You can modify the units used from here.