Hi, 

You can update SharePoint List Items using the "update Items" step on the workflow. You will need to provide the SharePoint Site URL and List name. You will then have all fields (columns) available and you can use the workflow variables or context fields to populate the fields. You can check our documentation here https://help.nintex.com/en-US/nwc/Content/Designer/Actions/SharePointOnline-UpdateItems.htm
Make sure you add a condition so that you update only the single item you want (ID = x).