Click Your Name | Setup | Company Profile | Manage Currencies.
Click Change Corporate.
Choose a new corporate currency from the list of active currencies, and click Save.
How to Enable Multiple Currencies in Salesforce?
Contact salesforce.com to request Multiple currencies.
Be prepared to provide the following information when writing a case:
The organization ID (production or sandbox)
The default currency stamp for current and future records (USD, EUR, GBP, etc.)
Confirmation that you understand that Multi-Currency can’t be disabled once enabled
Confirmation that you are a system administrator authorized on behalf of your organization to request Multi-Currency enablement and that you consent to the lockout of this organization for a certain period of time, depending on your organization’s data usage volume.
Where are multiple currencies mainly used in Salesforce?