Exchange rates change constantly. The revaluation adjusts your home currency values to match your foreign currency amounts at the current rate.
From the main menu, select Process, then Foreign Currency Revaluation.
Select the accounts to revalue: customers, suppliers, cash books, or any combination.
Select Next.
Select the Period for the revaluation.
Select Edit Exchange Rates.
Select the currency from the dropdown.
Set the exchange rate for that currency for the revaluation period.
Repeat for each currency you're revaluing.
Select OK.
In the Starting Reference field, enter your reference.
Select Next.
Print the Foreign Currency Variance Report.
If the report is correct, select Next.
Select Process.
Restore your backup from before the revaluation. See Restoring a backup. Then check your exchange rates and run the revaluation again.