Publish and roll back
Publishing is a click on the review page. Rolling back is another.
Publish
Click Publish on the review page. The upgrade becomes your live theme. On Pay per upgrade, this is when the $49 charge applies.
Your previous theme stays on your store as an unpublished theme.
Roll back
Click Roll back to the previous theme on the same page. Molt publishes the theme that was live before. The upgrade stays on your store as a draft, so you can change layouts and publish again.
Rolling back undoes the theme change. Anything you changed in the theme editor on the upgrade after publishing stays on the upgrade's copy.
Upgrades started from a draft
If you upgraded a draft theme, Molt leaves the result as a draft. Keep working on it in the theme editor, and publish it from your themes list when it's ready.