You can turn on the multicurrency feature if you have subscribed to QuickBooks Online (QBO) Plus and QBO Advance version. However, you cannot turn it off and your home currency will be fixed to USD. Here's how to turn on the multicurrency feature:
Go to the Gear icon and select Account and Settings.
Select Advanced from the left pane.
In the Currency section, click Edit ✎, and choose your Home Currency.
Tick the Multicurrency checkbox and confirm that you understand that you can't undo this option.