Inside the Ticket details page, click on the More button -> choose Edit to find the option to edit the requester's email address. 

The Edit option would not be available for the tickets with Source as Outbound email. To edit the requester of an Outbound email ticket, you would have to change the Source of the ticket first under ticket properties. 

If you are using Freshdesk on Mint, you will find the 'More' option against the subject of the ticket. Clicking here will give you an option to 'Edit' the details of the ticket.