I appreciate your time checking this with the Community.
I've checked a duplicate post of your concern on this link, where one of my peer added an answer about how the Revalue Currency Tool. You can check the steps she added and see if it works for you.
The market rate in QuickBooks Online is based on Wall Street On Demand foreign-exchange rates. Rates are automatically updating every four hours. Thus, when you adjust the currency, the system recalculates them based on a new rate. Here's a great article I've attached for your reference: What are Home Currency Adjustments?
Please let me know how this goes by clicking the Reply button below. I'm here if you need further assistance with QuickBooks. Have a great weekend!