1. On the menu bar select Settings then choose Currencies.
2. To change an exchange rate, select Exchange rate and enter the required rate.
3. To edit the details of a currency, select the currency then press Edit.
4. Amend as required then press OK.
5. Select Close.

The base currency is set when you first create your company in Sage Accounts. You can't change this. 

For the Euro exchange rate, you can only change the exchange rate, it's not possible to change any other details.