To change your billing currency, please send an email to billing@acme.com. However if your account is on a trial period, then you can change your currency from the portal by heading to the Subscriptions tab.
I can’t find my subscriptions tab?
This would be available on the left tab pane. This would only be accessible by the account admin of the account. If you do not see this panel you can check who the account admin is by moving to Admin > Agents and the agent's whose profile you are not able to edit would be the account admin.