To revert a subtitle to a previous revision you first need to navigate to the Talk’s page and choose the task language from the list on the left.
Then click on the Revision’s tab.

Choose the last revision and the revision you wish to revert the subtitles to, and click on Compare Revisions. 

You will now see a big red button on top of the translation or transcript. 

Click the red button to revert to a previous revision.

Video tutorial on How to roll back to a previous revision: