Multicurrency feature is available in QuickBooks Essentials and QuickBooks Plus only. This feature is not available in QuickBooks Simple Start.
To turn on Multicurrency feature in QBO:
From the home page screen > Gear icon (at upper right corner) > Your Company Settings > Account and Settings > Advanced > Currency > Multicurrency
Note: Once you turn on Multi-Currency, you can’t turn it OFF.
Also, check out our extensive multi-currency article: http://newqbo.com/multicurrency/
QuickBooks Multicurrency in QuickBooks Online US
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Also, check out this blog article for more info.