Salesforce: How to edit Dated Exchange Rates ?

Salesforce support multi-currency by default, but you need to request Salesforce Support to activate it and inform them the default corporate currency to use.

Then, you can enable Dated Exchange Rate to define exchange rate for every period (month, quarter or year).

Note: dated exchange rates are not currently used in forecasting, currency fields in other objects, or currency fields in other types of reports (see this article). So, make sure you keep updating Conversion rate in-sync with Dated exchange rates. Otherwise, forecast report will report different value with opportunity report (for example).

Once you enter the dated exchange rates, you will not see edit link/button in Active Currencies page.

To edit the rate, click the Currency Name link, it will show history of the rate, then click Edit link to change the rate, or Del link to delete exchange rate for a period.

Another way is to update using API, look for object DatedConversionRate with field name ConversionRate. You can use Developer Workbench to update this easily. Query the object and choose update from Id.
SELECT Id, IsoCode, ConversionRate, StartDate, NextStartDate FROM DatedConversionRate ORDER BY NextStartDate DESC

Reference: What should I know before enabling Advanced Currency Management with Historical Currency Exchange Rates?