Navigate to the mailing that you would like to add a new version to by selecting content > projects.
Open the campaign where the mailing sits by clicking on the campaign name, open the mailing by clicking on the mailing name.
Click on ‘add new version’ in the top right of the screen.
Fill in the version details and you can select a different template to the one selected for the mailing if necessary.
Hit create version once you have filled in the version details.
You will now see your new version listed with the other versions for this mailing.