Edit Selected SharepPoint Online Record with Nintex Automation Cloud

You'd need to get nintex forms for office 365 product vs using NAC. I'm pretty sure this isn't possible but id wait for a nintex employee to respond. 

Depending on what you're doing it is possible to display data (using data sources) from a selected item on a form (using a drop down with values of items from a list) and allowing user to edit. Then use workflow to update the item (update items action) in case changes were made.