Make sure that you don't have any data (not even the sample data) under Invoices, Items, Customers, Payments, Recurring Invoices, Credit Notes, Expenses and Customer Payments.

Note: Change option will be enabled, only if there is no data under any of the modules in your invoice account.

1. Login to Zoho Invoice and click the "Settings" tab.
2. Click the "Currencies" sub-link under "Currency & Tax Settings".
3. Click the "Change" link in the "Currency Settings" page.
4. Click the "New Currency" button to open the New Currency page.
5. Select your preferred base currency, from the "Currency Code" drop down box.
6. Select your preferred format, from the "Format" drop down box.
7. Click the "Save" button.