The import process has been simplified, especially for the update of existing records. To avoid confusion and to keep matters simple the App ID field will be used to uniquely identify each record.
The App ID is an out-of-the-box field available in all your apps, it is composed of 3 letters that are abbreviations of the App name (the first one given to the app) and 6 numbers, and it will look something like this: APP100001
If you have the App ID in your CSV file, the import process will recognize that record as existing and will update its data. If the App ID is not present in the file, then all the records there, are going to be created as entirely new ones.
This also changes the way in which you create relationships between your apps. Before you had to use the 'name' field to create that reference, but you could have many records with the same name and that caused some problems. Now you will use the App ID field to create these relationships.
You can find the App ID in each of your records and they will be included in all the files you export.
To learn how to update your data, please visit this tutorial.