'Data' tab > 'Connections' > data connection > 'Properties' > 'Definition' tab > 'Authentication Settings' button > 'None' radio button > OK > OK > Close.
With the Excel Services Authentication Settings set to None, Excel Services uses the unattended service account to refresh the data in the workbook after you have published it to SharePoint Server 2013.
Resources:
http://technet.microsoft.com/en-us/library/hh525344%28v=office.15%29.aspx
