Hello Stefanus,
you should just make the JSON structure pop-up a little bit wider if you can't see the Custom Name column and then you can click or double click on the names you want to edit.
Hi Stefanus
Make sure you are editing the JSON Structure document (), and not the Import Mapping document (
)
You can edit custom name in JSON Structure, and Import Mapping is only referencing that JSON Structure, so the Custom Name there is read-only