Hi there, @LilyLupin.

Thank you for visiting us here in the Community. I can share some insights about the currency exchange rate in QuickBooks Desktop.

Currently, the option to update the exchanging rates on existing transactions is unavailable. This is of course by design to maintain the integrity of your accounting data, otherwise, it would get scrambled regularly.  However, I know some applications you can integrate with QuickBooks to do this.

You can visit this link to have an overview of the applications you can use: https://quickbooks.intuit.com/apps/.

Another option is changing the amount of the bill to reflect the rate on the date you paid the transaction.

That should be enough information to get you on the right track, LilyLupin.

Let me know if you have other questions about the currency feature in QuickBooks. I'm still here to help you further. Have a good one!