Here are the steps to Import a Legacy Email Template into the current Email Builder.
Go to Nav > Marketing > Email & Broadcasts
Click Get Started
From the top right click Actions drop down and select Import Legacy Template
You can either scroll through the available templates or simply type the name of the template.
Click Convert template
Note: If your email was created though the "Classic Email Broadcast" it cannot be converted. You can either rebuild it in the current Email Builder or go to Nav > Marketing > Templates to create a template to convert. Either way you would need to rebuild the email.