Template content is not versioned, when creating your content any edits that you save cannot be reversed - you cannot undo. However, unless you publish the changes, the current published version (i.e. without your changes) will be the version sent to recipients.
If you publish changes the poster is updated. Any Templates that have been sent in the past will now display the published changes.
If you wish to retain the version previously sent, create a new template, copy the poster and then make your changes. Ensure you name the template appropriately. You can deactivate the old template, but once you do the recipients who have received the Template can no longer open the poster to view it.