how can I make a convertation in Zoho books from one currency to another?

For examle I have a list of Expenses and Invoices. According to it in my cash desk I have 600 eur, but I want to convert it in US Dollars (the main currency). Simply speaking I need minus 600 euro and plus 780 usd (1 eur=1.3 usd) in my cash desk. 

Thank you in advance.