Your billing currency is the default currency that you transact in, based on your billing country. This is also the currency in which any taxes applicable in your country (such as VAT) will be calculated.
To configure your billing currency, navigate to 'Billing and Payments' from the profile drop-down. On the 'Billing Information' tab, scroll to the bottom to select your billing currency from the drop-down.
Note: Due to regulations, the billing currency can't be changed once it has been configured and saved.
You can also add an alternative display currency to your invoices that can be edited at any stage.
Configure the billing currency when setting up Billing details for the first time.