How can I change the billing currency for the invoice?

Use the Manage Project Billing Information screen to make a change to the billing currency. You must recalculate the bill after this change has been made.

If you posted billing in one billing currency and must change the current billing to another currency, first run the Update Billing Currency Amounts toolkit. This toolkit restates the billing history into the selected new billing currency.  

Should I use this subtask if my billing currency and functional currency are the same?

You do not need to access this subtask if the billing and functional currencies are the same. The functional currency of the database automatically displays in the Billing Currency field and all exchange rates display as "1.0."