1, Make sure you have a back up of your workflow.
2, Brows to the Nintex workflows hidden folder (This is the URL of your site followed by /NintexWorkflows/Forms/AllItems.aspx)
3, Find Your workflow and open the folder.
4, there should be 3 files (Workflow Name, Workflow Name.xomal and Workflow Name.xomal.wfconfig) check them out.
5, Go to the version history of each of the files and restore to the version you want
6, Check in the 3 files.
7, Go into Nintex workflow designer and you should see that the workflow has changes back to the design that it was for that version (the version number will go up, it will not go back to the number that you are restoring it to)