Towards the end of a task's options area, you'll find a link labeled "Show version history".
Click it to see a summary of how the task has changed over time.
Use the "Restore this version" link for a prompt, confirming that you do in fact wish to restore this version. Once confirmed, Mechanic will update the task to reflect the state that it had as of that version. This creates a new version, too – the version history always reflects every change to a task, including restores.