I would like to know more about the objects that you are trying to upload to understand what objects would be updated.
For Instance, Im adding a Dashboard that contains 2 Visualizations and an index pattern that is used in the search of the data.
When I update the object
1 Dashboard
2 Visualizations
1 Index pattern that is used in the visualizations.
I selected Check for existing objects. In my case, I have an index pattern called wazuh-monitoring-*, so it will overwrite the index pattern and create new objects.
If you see the different objects,y ou can see the ID of each one, for instance, I imported again the same objects, creating new IDs, and you can see the different wazuh-monitoring-* index paterns

So in case you want to keep different objects (IDs) but with the same name you can select create new IDs. That will prevent any misconfiguration and backups, but if you want to overwrite because you have an existing one, you can select overwrite.