Your invoice currency is inherited from your project settings, which inherits its currency from your own company settings.  To change your default settings for the your account in the future, you must update your company default currency from settings.
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Any existing projects will need to have their settings updated as well.

You can update your invoice currencies on an invoice by invoice basis from the "Total" line of the invoice:
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However, we recommend setting your currencies on a per project basis so you don't have to remember to do it on each invoice. Learn how to do that here: http://help.hellobonsai.com/general-questions-and-overview/how-can-i-edit-my-project-details-like-currency-start-and-end-date-etc