Unfortunately, no quick way to change the template an approved email uses.
You can 'overwrite' the template with the new HTML for the new template.
But that means 'all' the emails that use this template will be the new look.
Thanks Rajesh. That's the only option I could think of..
If this is a one off, you can Replace the HTML of your email and disconnect it from any template.
In the Email go to Email Actions > HTML Tools > Replace HTML
I did hear some chat on the community that Maketo is putting fix out this year sometime to do this properly.
Or vote here: Allow templates to be changed after email created