1) To set fixed exchange rates inside SAP Business One first navigate to: Modules Tab > Administration > Exchange Rates and Indexes
2) From the Exchange Rates and Indexes make sure you have the Exchange Rates tab selected.
3) Choose Set Rate for Selection Criteria
4) Enter a date range (up to one month) for which you want to set the exchange rate.
5) Set the rate and click the checkboxes next to the rates you are defining.
6) Click OK.
7) The exchange rates you’ve set will now be copied to the Exchange Rates and Indexes screen. Click Update to apply the changes.