The exchange rate should be updated regularly from European Central Bank so should be completely accurate. There's no functionality to manually override currencies. I suppose you could try forcing it in the database, but your changes will likely be lost when it updates the exchange rates. Recommend confirming the exchange rate within CBSubs > Currencies.

But you can configure a multiplying factor in the CBSubs settings, first tab in the multicurrency, under "Markup of prices when not in currency of plan (in %)". That way, your conversion rate will always take in account your multi-currency extra-costs

> If I set a 5 in that field, it means there will be an increase of 5%

Yes, that's how it works. But best is to test it yourself to make sure it does the result you wish.