The currency in which your funds are displayed in is called your reporting currency. You can choose between SGD or USD reporting currency.
To change your reporting currency under account settings, simply click on Accounts > Settings > 'Change' under 'My reporting currency' > select the currency of your choice > click on the 'Save' button.
If I change my reporting currency now, will the currency in my past statements change too?
No, past statements will contain a record of your trades in the currency in which you performed them. Thus, changing your reporting currency will not affect your trade confirmations and monthly statements.