Hello - 

You cannot make changes to an instance of a workflow while it is in progress. However, if the workflow fails or completes, you can makes changes to the workflow, re-publish, and then resubmit the instance to run on the updated version of the workflow.

For more information on resubmitting a workflow, check out: https://help.nintex.com/en-US/nwc/Content/Dashboard/Instances.htm#Resubmit

Thanks