Currency is one of the parameters that is set automatically when you create your account.
If you at some point you'd want to change currency, you need to do the following:
navigate to the 'Settings' section in the 'Company' block in the menu on the left
at the top of the opened page you will see the 'Company settings" block. On the bottom of it, you can change the currency to the one you need and then save the changes.
