Your Portal's default currency can be set in the administration area in the "settings" menu.

If you don't see the currency you'd like to select, go to "lists" menu and find there the "currency" list, add a new one and make sure to enter a valid 3-letters ISO currency in the "Code" field (ie. GBP, EUR, USD)

Now go back to SETTINGS, select your new currency and update.

Currency will switch to default one once your session expires, you can anyway speed up this variation by resetting your cookies.