SAPbobsCOM.Company  company  =  new  SAPbobsCOM.Company();
company.Server  =  "server ip";
company.CompanyDB  =  "company db";
company.DbUserName  =  "db username";
company.DbPassword  =  "db password";
company.UserName  =  "sap user";
company.Password  =  "sap password";
company.DbServerType  =  SAPbobsCOM.BoDataServerTypes.dst_HANADB;
int  result  =  company.Connect();
if  (result  ==  0)
{
        SAPbobsCOM.SBObob  bo  =  company.GetBusinessObject(SAPbobsCOM.BoObjectTypes.BoBridge);
        bo.SetCurrencyRate("USD",  DateTime.Now,  rates["USD"],  true);
        company.Disconnect();
}
else
{
        Console.WriteLine(company.GetLastErrorDescription());
}

http://www.saphanadev.net/sap-b1/updating-currency-rates.html

Update exchange rates in SAP Business One

Updating Currency Rates In SAP Business One

C#