You can change your default currency by clicking on the Settings link at the top of the page. Then, hit the Invoice Details button in the sidebar on the right. Look for the pulldown menu labeled Currency. Click on that and choose from waaaaay more currencies than I can comfortably list in this space. I especially recommend the Estonia Krooni. That just sounds awesome.
Hit the Save Settings button at the bottom of the page when you’re done.
Of course, you can also change the currency for any particular invoice on the invoice itself.