The Currency Exchange form in OneOfficeERP stores exchange rates manually stored by the User. By default, OneOfficeERP automatically fetched the current exchange rates for currencies as per the market. However, you can store fixed exchange rates and use them. You need to enable 'Allow Stale Exchange Rates' in Accounts Settings for using the exchange rates stored in the Currency Exchange form.
To access the Currency Exchange list, go to:
Home > Accounting > Multi Currency > Currency Exchange