Under your home folder:
Library/Application Support/Calculator/FinancialRates.plist is the conversion table. You may want to trash that while Calculator is not running.
More calculator guts: your Library/Preferences/com.apple.calculator.plist, and the Calculator folder under your Library/Caches
The currency conversions in Calculator have been very odd lately. The update presents a gigantic dialog box with an apparently empty list of updated currencies, and then the From and To menus are padded with so much extra space that you can only see the names with the menus held down. Worse (this part could be a coffee deficiency speaking), the conversions appear to be reversed.