Can I modify the exchange rates manually?
Currency Converter+ does not allow custom exchange rates. On the contrary, we use reliable paid exchange rates providers to use the most accurate exchange rates possible.
There are several apps in the Shopify App Store that offer the option to set up custom rates, you can look for them at https://apps.shopify.com/search/query?q=currency
However, please note that they only convert prices on the web pages, so the customer will still see and pay the same amount on checkout, regardless of the currency selected.