To manage the Country and Regional Settings for your Account, navigate to your Settings and select "General Settings". The first section on this page is your Country and Regional Settings:
Here, you can set the Country associated to your account, which will automatically set a Currency against your account. You can also change your Locale, Time Zone, and Date Format:
Please note that if you are integrated with an Accounting Provider you will be unable to manage the Country or Currency associated to your account here, as those must be managed through your Accounting Provider.
Please also note that we do not currently support having more than one currency set against an account. We are looking in to adding this functionality in the future.