The default behavior in Freshdesk is that the reply to a ticket would be sent using the support email address from which that ticket was created. 

However, if your requirement is to reply through an email address irrespective of the email address from which the ticket was created, you could install an app called "Customize 'From' Email Address". This can be installed under Admin > Support Operations > Apps > Get More Apps.

Here is a sample screenshot of the app settings.