Katana allows you to edit the currency conversion rate after it has been fixed so you can modify and recalculate the converted total if needed.
This is usually required if you need to match exchange rates between two systems or to match with the currency conversion rate of the actual payment that was made.
Read here about when Katana fixes your currency rate.
Editting currency rate on a Sales order (SO)
Open a SO with a foreign currency as order currency. When the currency rate is fixed, you'll see an Edit rate link.
Click on Edit rate under the Totals section of the SO and edit the Currency rate or Fixed at date.
NOTE: Changing the fixed date will not retrieve the currency rate for that past date and will not update the currency rate to your selected date.
After changing the conversion rate the Converted Total (Base Currency) will be recalculated.
Why can't you change the conversion rate?
When an order has already been submitted to accounting as an invoice or bill (via accounting integrations), it's no longer possible to change the rate in Katana. This should be done in your accounting platform.