How to revaluation of forex currency in bank account , and not affected original amount?

I'm happy to see you here in the Community, @elaine20.

I've got the steps that you need in creating a currency revaluation in QuickBooks Online.

The system allows you to keep track of the changes with the exchange rates on your foreign balances. That said, you have the option to create a home currency adjustment. Please know that this option is only available to the Essentials and Plus version of QuickBooks.

To do that:

1. Go to Gear icon at the top.
2. Under Lists select on Currencies.
3. Find the currency you want to adjust.
4. From the Actions column, select the small arrow ▼ icon.
5. Click on Revalue Currency.
6. Select a date (today or a day in the past) to run a currency revaluation.
7. Choose whether you'd like the revaluation to be based on the market exchange rate or a rate you specify.
8. Select the account(s) you want to apply the revaluation to.
9. Hit on Revalue.

For complete details about this process, please see this link: Enter home currency adjustments for your foreign balances.

To learn more on how multi-currency works in QuickBooks Online and to guide you in fixing its adjustments, you can check out these sources below: 

• How Multicurrency works 
• Frequently Asked Questions about Home Currency Adjustments

If there's anything that I can assist you with, let me know. I'll be around to help. Have a lovely day!