Every organization will have a default currency set in https://donorbox.org/org/edit. Set this to your preferred default currency, as shown:
For each individual donation campaigns, you can change the currency to be different from your default currency. To do this, click on the Amounts tab of your form editor.
Under Base Currency, select your form's main currency. You can also add other currency options for donors to select their preferred currency, in case it differs from yours.
Click Save or Update when you're done.