I have already reconciled the previous month and unfortunately deleted or adjusted or changed the reconciled transactions? How could I correct that error?

I'll share some details and where you can edit the rate, @amagtaas-ehrsgro.

In QuickBooks Online, the exchange rate is always calculated as the number of Home currency units that equal 1 Foreign currency unit. To view the currency information, you can follow the steps below. Please note that you should not modify exchange rates from the Currency page unless it is to be used for an entire day.

1. Go to the Gear > Currencies.
2. Find the currency and click the Edit currency exchange link under the Action column.
3. Select a date to see the exchange rate. 
4. Click Save.

For more information about exchange rate, you can read these articles:

• About exchange rates
• Enter an exchange rate

If you have any additional questions about the exchange rate, or other concerns with QuickBooks Online, please let me know in the comments below. I'm here to help you out again.