If the data you are receiving has known (preferably unique) values you could use them to do a Retrieve with an XPath Constraint on the data already in your application. If this returns a match, you can use a Change Object action to update the attributes you are interested in. If it doesn't return a match, then it is a new item and you can use a Create Object action to add the new data.
https://docs.mendix.com/refguide/retrieve/#333-xpath-constraint
I hope this helps.
You can iterate the list and change the attributes including associations.