I am hoping someone can help me. I have searched and searched but all the answers I find relate to older version of SharePoint and Nintex. 

I currently have a list set up that has a workflow that takes the item that's entered and puts it on a calendar. My issue is whenever we do a modify from the list, instead of updating the entry on the calendar, it creates a new calendar event. When I try to use the suggestions from the posts from older version, I don't have access to the options that are listed. I have also tried assigning an item ID to the entries using GUID and Integer (separately, not at the same time), but when I do this, the number is the same on all entries. 

The original workflow is pretty simple, but attaching pictures for reference.