Tapping a foreign currency in the Currencies Screen lists all exchange rates that you have entered as well as those obtained from the web.
The list shows the following:
Exchange rates obtained from the web are removed from the list when they are no longer needed. Typically exchange rates are kept in the list for about a week. This only applies to rates supplied from the web; rates you enter yourself are only deleted if you choose to delete them yourself.
Tap Add a new exchange rate shown at the bottom of the list.
Tap the rate in the list. The list is ordered by date.
Swipe the rate to reveal the Delete button. Tap Delete to remove the rate.