Hi Linhphamtr,
Welcome to Community! We're glad to see you here! It's great to hear that you're taking advantage of the multi currency option that QuickBooks Online offers. When reconciling your account, you'll have the ability to make changes to the exchange rate as necessary to ensure your books remain balanced. I'd be happy to assist!
To revalue the currency, follow these steps;
1. Go to the Gear icon in the top right of the dashboard
2. Select Currencies
3. Click on the drop-down arrow to Revalue the Market Rate by date, or enter a Custom Rate
That's it! You're all set to continue with reconciling your account!
Should you have any additional questions, please feel free to reach back out. We'd be glad to help!